restapi: Update VM - Validate Against Empty Domain (#1049080)

When updating VM, if <domain> supplied, verify that
<domain><name> supplied as well.

Bug-Url: http://bugzilla.redhat.com/1049080
Change-Id: If0555c4a26e4fe99058d490a3f5808248683a1fa
Signed-off-by: Ori Liel <oliel@redhat.com>