block/dirty-bitmap: add bdrv_dirty_bitmap_merge_internal

block/dirty-bitmap: add bdrv_dirty_bitmap_merge_internal

I'm surprised it didn't come up sooner, but sometimes we have a +busy
bitmap as a source. This is dangerous from the QMP API, but if we are
the owner that marked the bitmap busy, it's safe to merge it using it as
a read only source.

It is not safe in the general case to allow users to read from in-use
bitmaps, so create an internal variant that foregoes the safety
checking.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190709232550.10724-10-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>