MAINTAINERS: Add qemu-progress to the block layer

util/qemu-progress.c is currently unmaintained. The only user of its
functionality is qemu-img, so it effectively is part of the block layer.

Suggested-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170428165517.30341-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>