Add helper to populate vhost-user message regions

Add helper to populate vhost-user message regions

When setting vhost-user memory tables, memory region descriptors must be
copied from the vhost_dev struct to the vhost-user message. To avoid
duplicating code in setting the memory tables, we should use a helper to
populate this field. This change adds this helper.

Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <1588533678-23450-2-git-send-email-raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>