MAINTAINERS: Add Leonardo and Peter as reviewers

Now that David has stepped down with Migration maintainership,
Leonardo and Peter has volunteer to review the migration patches.
This way they got CC'd on every migration patch.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Leonardo Bras <leobras@redhat.com>

MAINTAINERS

@@ -3154,6 +3154,8 @@ F: scripts/checkpatch.pl
 
 Migration
 M: Juan Quintela <quintela@redhat.com>
+R: Peter Xu <peterx@redhat.com>
+R: Leonardo Bras <leobras@redhat.com>
 S: Maintained
 F: hw/core/vmstate-if.c
 F: include/hw/vmstate-if.h