core: allow hibernate for vm without image disks (#928708)

remove check for image disks,
move storage domain space check to storage domain selection method

Change-Id: I83a5ffc4653929c65174e27eae3e605dbef75c4c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=928708
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>