block: Keep "driver" in bs->options Instead of passing a separate drv argument to bdrv_open_common(), just make sure that a "driver" option is set in the QDict. This also means that a "driver" entry is consistently present in bs->options now. This is another step towards keeping all options in the QDict (which is the represenation of the blockdev-add QMP command). Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com>