core: PerformImagesChecks no check for no disks

Removed the check for no image disks from PerfomImagesChecks: creating a
template, creating a snapshot and removing a snapshot should also work
on diskless VMs, however useless it may be.

Move VM has this check explicitly defined in its canDoAction, and is
deprecated, regardless.

After this change, there is no difference between PerformImagesChecks
and checkDiskImages, so the ltater (which was private) was removed,
and the code inlined into PerformImagesChecks.

Change-Id: Ib4638c8dc5ba6990cdf0b335c76645ee9dedbeee
Signed-off-by: Allon Mureinik <amureini@redhat.com>
core: Remove ImagesHandler.PerformImagesChecks

After the recent changes, PerformImagesChecks had the following
functionality:
For each disk:
- GetImageInfo from VDSM
- if it does not exist, fail
- if it's status is different from the engine's status, update the
  engine's status.

Analyzing ImageStatus' different values, this validation is useless:
- OK: The engine did not issue any command regarding this image. It may
  have been manually removed from the storage itself, but that could
  also happen right after the check finishes successfully.
- LOCKED: The engine is in the middle of performing a command on the
  image. It's status in VDSM is meaningless, as it may change any moment
  now.
- ILLEGAL/INVALID: The engine issued a command on the image, and it was
  interrupted somehow (e.g., restart, network failure, etc.). The image
  may have been partly or fully compromised, and it cannot be trusted
  anyway.

In conclusion, this validation NEVER returns any useful information, and
thus could (and should!) be removed.
For good measures, an additional validation that the disks aren't
ILLEGAL in the engine side was added to the commands where it was
missing, just to be safe.

Change-Id: I983a4c3a016909385ec16f21803ef3a82e4dcaf6
Signed-off-by: Allon Mureinik <amureini@redhat.com>