blockdev: Split off basic bitmap operations for qemu-img

Upcoming patches want to add some basic bitmap manipulation abilities
to qemu-img.  But blockdev.o is too heavyweight to link into qemu-img
(among other things, it would drag in block jobs and transaction
support - qemu-img does offline manipulation, where atomicity is less
important because there are no concurrent modifications to compete
with), so it's time to split off the bare bones of what we will need
into a new file block/monitor/bitmap-qmp-cmds.o.

This is sufficient to expose 6 QMP commands for use by qemu-img (add,
remove, clear, enable, disable, merge), as well as move the three
helper functions touched in the previous patch.  Regarding
MAINTAINERS, the new file is automatically part of block core, but
also makes sense as related to other dirty bitmap files.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200513011648.166876-6-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>