util/iov: Introduce iov_send_recv_with_flags()

util/iov: Introduce iov_send_recv_with_flags()

In order to support a new update function, a flag needs to be passed
to the kernel via the socket send call (MSG_MORE) to notify it that
more data is to be expected to calculate the hash correctly.

Add a new iov helper for this purpose.

Signed-off-by: Alejandro Zeise <alejandro.zeise@seagate.com>
[ clg: - Split iov changes from original patch
       - Checkpatch fixes ]
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>