core: Extract Validator creation and apply usages

There are several places creating a MultipleStorageDomainsValidator
based on a VM and DisksImage List in the same manner.
Some new code for bellow bzs does the same, as will other patches. This
patch removed future code duplicity by extracting the
duplicated code into VmCommand.

Change-Id: I3a6d04fe1a670c814b44bad5304a894faa8f8510
Bug-Url: https://bugzilla.redhat.com/1054175
Signed-off-by: Vered Volansky <vvolansk@redhat.com>