MAINTAINERS: Update for migrated migration code

My previous patches migrated the migration code into migration/
but didn't update MAINTAINERS.

Note that does mean that the owner for block-migration.c
changes, but I'll ask block people what they want to do.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>