tools/libxs: Use writev()/sendmsg() instead of write()

tools/libxs: Use writev()/sendmsg() instead of write()

With the input data now conveniently arranged, use writev()/sendmsg() instead
of decomposing it into write() calls.

This causes all requests to be submitted with a single system call, rather
than at least two.  While in principle short writes can occur, the chances of
it happening are slim given that most xenbus comms are only a handful of
bytes.

Nevertheless, provide {writev,sendmsg}_exact() wrappers which take care of
resubmitting on EINTR or short write.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>