core: minor cleanups

* Remove _destinationVds from RunVmCommandBase and put relevant fields
  in RunVmCommand & MigrateVmCommand. It is better in terms of
  encapsulation since they are now private

* Refactor RunVmCommand#getDestinationVds to be more readable

* Remove MigrateVmCommand#forcedMigrationForNonMigratableVM

* Remove explicit usages of DbFacade class within MigrateVmCommand

* Few renames

Change-Id: I2fa22569d9195f69cec7fa9c0a4931b2975e41ec
Signed-off-by: Arik Hadas <ahadas@redhat.com>