dirty-bitmap: Change bdrv_dirty_bitmap_size() to report bytes

dirty-bitmap: Change bdrv_dirty_bitmap_size() to report bytes

We're already reporting bytes for bdrv_dirty_bitmap_granularity();
mixing bytes and sectors in our return values is a recipe for
confusion.  A later cleanup will convert dirty bitmap internals
to be entirely byte-based, but in the meantime, we should report
the bitmap size in bytes.

The only external caller in qcow2-bitmap.c is temporarily more verbose
(because it is still using sector-based math), but will later be
switched to track progress by bytes instead of sectors.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>