Introduce VIR_JOB_MIGRATION_SAFE job type

Introduce VIR_JOB_MIGRATION_SAFE job type

This is a special job for operations that need to modify domain state
during an active migration. The modification must not affect any state
that could conflict with the migration code. This is useful mainly for
event handlers that need to be processed during migration and which
could otherwise time out on acquiring a normal MODIFY job.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>