ubuntu_nbd_smoke_test failed with B-AWS on some instances

Like bug 1803120, this test was found failing with the following nodes:

AMD64:
 - t2.small
 - c5.large

ARM64
 - a1.large
 - a1.meduim

It looks like they're all having 8G disk space. Probably it's not suitable for this test.

 Image path: /tmp/nbd_image.img
  Mount point: /mnt/nbd-test-7113
  Date: Tue Mar 12 10:25:17 UTC 2019
  Host: ip-172-31-2-218
  Kernel: 4.15.0-1034-aws #36-Ubuntu SMP Tue Mar 5 23:17:16 UTC 2019
  Machine: ip-172-31-2-218 x86_64 x86_64
  CPUs online: 1
  CPUs total: 1
  Page size: 4096
  Pages avail: 285053
  Pages total: 509930
  Free space:
  Filesystem Size Used Avail Use% Mounted on
  udev 985M 0 985M 0% /dev
  tmpfs 200M 800K 199M 1% /run
  /dev/xvda1 7.7G 1.9G 5.9G 24% /
  tmpfs 996M 0 996M 0% /dev/shm
  tmpfs 5.0M 0 5.0M 0% /run/lock
  tmpfs 996M 0 996M 0% /sys/fs/cgroup
  /dev/loop1 18M 18M 0 100% /snap/amazon-ssm-agent/930
  /dev/loop0 91M 91M 0 100% /snap/core/6350
  tmpfs 200M 0 200M 0% /run/user/1000
  --------------------------------------------------------------------------------

  NBD device /dev/nbd0 created
  found nbd export
  NBD exports found:
  test
  starting client with NBD device /dev/nbd0
  Negotiation: ..size = 128MB
  creating ext4 on /dev/nbd0
  mkfs on /dev/nbd0 succeeded after 0 attempt(s)
  checking ext4 on /dev/nbd0
  fsck from util-linux 2.31.1
  /dev/nbd0: clean, 11/32768 files, 9787/131072 blocks

  mount:
  /dev/nbd0 on /mnt/nbd-test-7113 type ext4 (rw,relatime,data=ordered)
  mounted on /dev/nbd0

  free:
  Filesystem 1K-blocks Used Available Use% Mounted on
  /dev/nbd0 122835 1550 112111 2% /mnt/nbd-test-7113

  creating large file /mnt/nbd-test-7113/largefile
  -rw-r--r-- 1 root root 100M Mar 12 10:25 /mnt/nbd-test-7113/largefile

  free:
  Filesystem 1K-blocks Used Available Use% Mounted on
  /dev/nbd0 122835 103951 9710 92% /mnt/nbd-test-7113

  removing file /mnt/nbd-test-7113/largefile
  unmounting /mnt/nbd-test-7113
  stopping client
  disconnect, sock, done
  Found kernel warning, IO error and/or call trace
  echo
  [ 250.567644] creating backing nbd image /tmp/nbd_image.img
  [ 252.429142] NBD device /dev/nbd0 created
  [ 254.120378] found nbd export
  [ 255.171532] starting client with NBD device /dev/nbd0
  [ 255.176674] creating ext4 on /dev/nbd0
  [ 255.352573] mkfs on /dev/nbd0 succeeded after 0 attempt(s)
  [ 255.385827] checking ext4 on /dev/nbd0
  [ 255.403795] EXT4-fs (nbd0): mounted filesystem with ordered data mode. Opts: (null)
  [ 255.413695] mounted on /dev/nbd0
  [ 255.415667] creating large file /mnt/nbd-test-7113/largefile
  [ 256.285711] removing file /mnt/nbd-test-7113/largefile
  [ 256.300090] unmounting /mnt/nbd-test-7113
  [ 257.322408] stopping client
  [ 257.324768] block nbd0: NBD_DISCONNECT
  [ 257.325170] block nbd0: shutting down sockets
  [ 257.325926] nbd0: detected capacity change from 0 to 134217728
  [ 257.325953] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.330021] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.335199] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.339034] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.343980] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.347993] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.353198] ldm_validate_partition_table(): Disk read failed.
  [ 257.353211] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.357324] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.362279] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.366225] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.371191] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.375203] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.380268] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.384291] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.389375] Dev nbd0: unable to read RDB block 0
  [ 257.392948] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.396889] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.402048] print_req_error: I/O error, dev nbd0, sector 24
  [ 257.406092] Buffer I/O error on dev nbd0, logical block 3, async page read
  [ 257.411047] print_req_error: I/O error, dev nbd0, sector 0
  [ 257.414908] Buffer I/O error on dev nbd0, logical block 0, async page read
  [ 257.420191] nbd0: unable to read partition table
  [ 257.586991] Found kernel warning, IO error and/or call trace
  [ 257.587022] echo
  killing server