tools/virtio: xen stub

Fixes test module build.
Reported-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/xen/xen.h

+#ifndef XEN_XEN_STUB_H
+#define XEN_XEN_STUB_H
+
+#define xen_domain() 0
+
+#endif