block: do not reuse the backing file across bdrv_close/bdrv_open This is another bug caused by not doing a full cleanup of the BDS across close/open. This was found with mirroring by Shaolong Hu, but it can probably be reproduced also with eject or change. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>