core: HibernateVmCommand canDoAction cleanup

return canDoAction on failed check instead of if-else,
move storage domains check to be last,
removed useless call to update vm disks,
removed useless comment.

Change-Id: Ib2c7682a7b1b47e84f71fe1a6f91b3af78d799ab
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>