qcow2-bitmap: make bytes_covered_by_bitmap_cluster() public

Rename bytes_covered_by_bitmap_cluster() to
bdrv_dirty_bitmap_serialization_coverage() and make it public.
It is needed as we are going to share it with bitmap loading in
parallels format.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Message-Id: <20210224104707.88430-2-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>