InterfaceManager: unmap devices from tmp mapping

The unmap_if method now checks if the if_id is present in _id_mapping or
_tmp_mapping.

Even though this method should only be called in the cleanup phase of a
test run when every interface should have been moved to _id_mapping this
patch should prevent exceptions being thrown in unexpected scenarios.

Signed-off-by: Ondrej Lichtner <olichtne@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>