Engine: VMs search by name NPE

The search is trying to search for OS which is not a string.
After commit: 282f87b3b0a96008d87cad21cc48e4d8adcd4cf9 change
VmOsType with OS_info

Change-Id: I9eb009b6463f33b37984f3f1410e5c47c6159911
Signed-off-by: Shahar Havivi <shavivi@redhat.com>