vhost-user: factor out "vhost_user_write_sync"

vhost-user: factor out "vhost_user_write_sync"

The tails of the "vhost_user_set_vring_addr" and "vhost_user_set_u64"
functions are now byte-for-byte identical. Factor the common tail out to a
new function called "vhost_user_write_sync".

This is purely refactoring -- no observable change.

Cc: "Michael S. Tsirkin" <mst@redhat.com> (supporter:vhost)
Cc: Eugenio Perez Martin <eperezma@redhat.com>
Cc: German Maglione <gmaglione@redhat.com>
Cc: Liu Jiang <gerry@linux.alibaba.com>
Cc: Sergio Lopez Pascual <slp@redhat.com>
Cc: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Albert Esteve <aesteve@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20231002203221.17241-4-lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>