Callers can create an iterator of meta bitmap with
bdrv_dirty_meta_iter_new(), then use the bdrv_dirty_iter_* operations on
it. Meta iterators are also counted by bitmap->active_iterators.

Also add a couple of functions to retrieve granularity and count.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1476395910-8697-11-git-send-email-jsnow@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>