xfs_io: Disable -V if pwritev is not available

xfs_io: Disable -V if pwritev is not available

This results in default case which will display the error message
that command is not found. This is required for xfs_io builds which
did not support pwritev and an attempt is made to use -V.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
[sandeen@sandeen.net: restrict default: printf to ifdef'd options]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>