vm seems to hang on kvm

Hello,

we have 4 linux guest VMs running on ubuntu 14.04 with 3.13.0-35-generic #62-Ubuntu SMP kernel. From time to time the VMs seems to hang for about 30 seconds, and afterwards are continuing their operations.
I cannot find anything in the syslog files on the host nor on the guests.
Performance parameters collected by sar (10 minutes sampling) only shows a slight iowait appearing on the host:

12:05:01 PM CPU %user %nice %system %iowait %steal %idle
12:15:01 PM all 3.16 0.00 0.55 0.00 0.00 96.29
12:25:01 PM all 3.34 0.00 0.73 0.00 0.00 95.93
12:35:01 PM all 3.65 0.00 0.94 1.44 0.00 93.97 <----- iowait is 1.44 the only value different than 0 for the whole day
12:45:01 PM all 3.27 0.00 0.65 0.00 0.00 96.08
12:55:01 PM all 3.18 0.00 0.58 0.00 0.00 96.24

The only disk based fs is the / :
$ df -h
Filesystem Size Used Avail Use% Mounted on
udev 63G 12K 63G 1% /dev
tmpfs 13G 1.4M 13G 1% /run
/dev/sda1 275G 13G 249G 5% /
none 4.0K 0 4.0K 0% /sys/fs/cgroup
none 5.0M 0 5.0M 0% /run/lock
none 63G 12K 63G 1% /run/shm
none 100M 0 100M 0% /run/user

while the sar values for the disk does not show anything unusual:
12:05:01 PM DEV tps rd_sec/s wr_sec/s avgrq-sz avgqu-sz await svctm %util
12:15:01 PM dev8-0 0.79 0.00 25.20 31.76 0.00 0.00 0.00 0.00
12:25:01 PM dev8-0 0.80 0.00 25.52 31.90 0.00 0.00 0.00 0.00
12:35:01 PM dev8-0 0.76 0.01 25.15 33.18 0.00 0.01 0.01 0.00
12:45:01 PM dev8-0 0.79 0.00 25.01 31.46 0.00 0.01 0.01 0.00
12:55:01 PM dev8-0 0.80 0.00 25.84 32.44 0.00 0.00 0.00 0.00
Average: dev8-0 0.79 0.00 25.34 32.14 0.00 0.00 0.00 0.00

The VMs have their discs on a ceph cluster and are accessing them using librbd.
I can see some traffic peak on the storage interface:
12:05:01 PM IFACE rxpck/s txpck/s rxkB/s txkB/s rxcmp/s txcmp/s rxmcst/s %ifutil
12:15:01 PM vlan6 157.49 148.81 42.45 328.90 0.00 0.00 0.00 0.00
12:25:01 PM vlan6 154.82 148.44 41.97 327.32 0.00 0.00 0.00 0.00
12:35:01 PM vlan6 157.22 154.34 47.12 505.42 0.00 0.00 0.00 0.00 <----- txkB goes upt to 505 from an average of 328
12:45:01 PM vlan6 152.60 147.00 41.15 319.85 0.00 0.00 0.00 0.00
12:55:01 PM vlan6 156.09 147.38 42.22 323.50 0.00 0.00 0.00 0.00
Average: vlan6 155.64 149.19 42.98 361.00 0.00 0.00 0.00 0.00

Unfortunately I don't have access to the VMs. I just got the sar files + the system log (/var/log/messages - the guests are centos 7), but I cannot see anything unusual in them at the given period.

Does anybody has an idea what else can I check?
Is it possible that the iowait has appeared due to the increased network traffic? (AFAIK only fs related syscalls are included in that metric).

Any suggestion is welcome.

Kind regards,
Laszlo