core: VmPoolCommandBase: Direct domain validation

Stopped using ImagesHandler to validate the storage domain in
VmPoolCommandBase, and instead directly called StorageDomainValidator.

This is done as a step in the ongoing effort to decouple
ImagesHandler.PerformImagesChecks, and remove irrelevant validations
(e.g., storage domains) from it.

Change-Id: I7d1854bc4a33f2f7ea218670082dd9bee7467cd9
Signed-off-by: Allon Mureinik <amureini@redhat.com>