migration: Add co-maintainers for migration

Per the qemu upstream call a few hours ago, proposing Fabiano and myself as
the co-maintainer for migration subsystem to help Juan.

Cc: Fabiano Rosas <farosas@suse.de>
Cc: Juan Quintela <quintela@redhat.com>
Acked-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231003143847.9245-1-peterx@redhat.com>

MAINTAINERS

@@ -3207,7 +3207,8 @@ F: scripts/checkpatch.pl
 
 Migration
 M: Juan Quintela <quintela@redhat.com>
-R: Peter Xu <peterx@redhat.com>
+M: Peter Xu <peterx@redhat.com>
+M: Fabiano Rosas <farosas@suse.de>
 R: Leonardo Bras <leobras@redhat.com>
 S: Maintained
 F: hw/core/vmstate-if.c