MAINTAINERS: add Stefan Hajnoczi as block and virtio-blk co-maintainer

Kevin has requested co-maintainership to give him more time to write
code.  We will alternate patch review duties on a weekly basis.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 8f5681f..2ede20d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -553,6 +553,7 @@
 
 virtio-blk
 M: Kevin Wolf <kwolf@redhat.com>
+M: Stefan Hajnoczi <stefanha@redhat.com>
 S: Supported
 F: hw/virtio-blk*
 
@@ -583,6 +584,7 @@
 
 Block
 M: Kevin Wolf <kwolf@redhat.com>
+M: Stefan Hajnoczi <stefanha@redhat.com>
 S: Supported
 F: block*
 F: block/