MAINTAINERS: Add Fam Zheng as a co-maintainer of block I/O path

As agreed with Stefan, I'm listing myself a co-maintainer of block I/O
path and assist with the maintainership.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-id: 1459849105-7767-1-git-send-email-famz@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

MAINTAINERS

@@ -998,6 +998,7 @@ T: git git://repo.or.cz/qemu/kevin.git block
 
 Block I/O path
 M: Stefan Hajnoczi <stefanha@redhat.com>
+M: Fam Zheng <famz@redhat.com>
 L: qemu-block@nongnu.org
 S: Supported
 F: async.c