MAINTAINERS: Block layer core, qcow2 and blkdebug

As agreed with Kevin and already practiced for a while, I am adding
myself as co-maintainer of the block layer core, qcow2 and blkdebug.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>