core: Remove vmId from PerformImagesChecks

Removed the VM ID from PerformImagesChecks, Since this function is
all about validating images, not VMs.

In order to achieve this support for passing a null list of disks was
dropped, and in the one(!) place it was used (RemoveSnaphotCommand),
an explicit DAO call was added.

Change-Id: Ia628932ca7767aa9cca4b572523321346828dfa0
Signed-off-by: Allon Mureinik <amureini@redhat.com>