core: VmRunHandler: correct check for disk images

VmRunHandler has a series of checks on VM images - these checks should
only be performed if the VM has image based disks, not disks in general
(which could be LUN disks).

Change-Id: Ib47cf77154a5e46af23a10550884bbd6cdeecb30
Signed-off-by: Allon Mureinik <amureini@redhat.com>