vhost-user: hoist "write_sync", "get_features", "get_u64"

vhost-user: hoist "write_sync", "get_features", "get_u64"

In order to avoid a forward-declaration for "vhost_user_write_sync" in a
subsequent patch, hoist "vhost_user_write_sync" ->
"vhost_user_get_features" -> "vhost_user_get_u64" just above
"vhost_set_vring".

This is purely code movement -- 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: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Albert Esteve <aesteve@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20231002203221.17241-6-lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>