MAINTAINERS: add block driver sub-maintainers

MAINTAINERS: add block driver sub-maintainers

There are a number of contributors who maintain block drivers (image
formats and protocols).  They should be listed in the MAINTAINERS file
so that get_maintainer.pl lists them.

Note that commits are still merged through Kevin or Stefan's block tree
but the block driver sub-maintainers are usually the ones to review
patches.

Acked-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>
Acked-by: Stefan Weil <sw@weilnetz.de>
Acked-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Jeff Cody <jcody@redhat.com>
Acked-by: Anthony Liguori <aliguori@amazon.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>