core: change import vm to be VmCommand - part 2

Inline checkStoragePool method call and moved fillMacAddress method, to
reduce the coupling between ImportVmCommand and
MoveOrCopyTemplateCommand.

Change-Id: Ie7063d22fbe4bd81de24f050660e35b3ea7b96cd
Signed-off-by: Arik Hadas <ahadas@redhat.com>