MAINTAINERS: make block I/O path Stefan Hajnoczi's responsibility

The block I/O path includes the asynchronous I/O machinery and
read/write/flush/discard processing.  It somewhat arbitrarily also
includes block migration, which I've found myself reviewing patches for
over the years.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>