xen: remove the config file after migration

xen: remove the config file after migration

Since it's not good to have the instance definition file left on the
source one if the migration succeeded, we remove it after the ‘xm
migrate’ call.

In order to do this using the existing self._RemoveConfigFile, we change
the protocol for this method to take the instance name and not instance
object as parameter, as in the migration we don't have the instance
object (and the only other caller is cheap to modify).

Reviewed-by: imsnah