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: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

MAINTAINERS

@@ -553,6 +553,7 @@ T: git git://github.com/kvaneesh/QEMU.git
 
 virtio-blk
 M: Kevin Wolf <kwolf@redhat.com>
+M: Stefan Hajnoczi <stefanha@redhat.com>
 S: Supported
 F: hw/virtio-blk*
 
@@ -583,6 +584,7 @@ F: audio/
 
 Block
 M: Kevin Wolf <kwolf@redhat.com>
+M: Stefan Hajnoczi <stefanha@redhat.com>
 S: Supported
 F: block*
 F: block/