vvfat: Fix inherit_options flags

Overriding flags violates the precedence rules of
bdrv_reopen_queue_child. Just like the read-only option, no-flush should
be put into the options. The same is done in bdrv_temp_snapshot_options.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>