Engine: pass default displays head as string

New VM store the heads value as integer, which pass to vdsm to treat it
as String.

Change-Id: I938e7a9285282f72c6b08b23b804cd84f6f8d060
Bug-Url: https://bugzilla.redhat.com/1018760
Signed-off-by: Shahar Havivi <shavivi@redhat.com>