vringh: add vringh_kiov_length() helper

This new helper returns the total number of bytes covered by
a vringh_kiov.

Suggested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210315163450.254396-7-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>