qcow2: Release read-only bitmaps when inactivated

qcow2: Release read-only bitmaps when inactivated

During migration, we release all bitmaps after storing them on disk, as
long as they are (1) stored on disk, (2) not read-only, and (3)
consistent.

(2) seems arbitrary, though.  The reason we do not release them is
because we do not write them, as there is no need to; and then we just
forget about all bitmaps that we have not written to the file.  However,
read-only persistent bitmaps are still in the file and in sync with
their in-memory representation, so we may as well release them just like
any R/W bitmap that we have updated.

It leads to actual problems, too: After migration, letting the source
continue may result in an error if there were any bitmaps on read-only
nodes (such as backing images), because those have not been released by
bdrv_inactive_all(), but bdrv_invalidate_cache_all() attempts to reload
them (which fails, because they are still present in memory).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200730120234.49288-2-mreitz@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>