When BDSs are created by qemu itself (e.g. as filters in block jobs),
they may not have a "driver" option in their options QDict.  When
generating a json:{} filename, however, it must always be present.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190201192935.18394-31-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>