vvfat: Fix inherit_options flags

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 <[email protected]>
Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>