xfs_io: implement pwritev for vectored writes

xfs_io: implement pwritev for vectored writes

When looking at KVM based direct IO patterns, I noticed that it was
using preadv and pwritev, and I could not use xfs_io to simulate
these IO patterns. Extend the pwrite command to be able to issue
vectored write IO to enable use to simulate KVM style direct IO.

Also document the new parameters as well as all the missing pwrite
command parameters in the xfs_io(8) man page.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Mark Tinguely <tinguely@sgi.com>