When bdrv_close_all() is called, instead of force-closing all root
BlockDriverStates, it is better to just drop the reference from all
BlockBackends and let them be closed automatically. This prevents BDS
from getting closed that are still referenced by other BDS, which may
result in loss of cached data.

This patch adds a function for doing that, but does not yet incorporate
it in bdrv_close_all().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>