libxl: Fix xl shutdown for HVM without PV drivers

A return value of zero means no PV drivers. Restore a hunk which was removed.

Fixes commit b183e180bc

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wl@xen.org>

tools/libxl/libxl_domain.c

@@ -774,6 +774,9 @@ int libxl__domain_pvcontrol(libxl__egc *egc, libxl__xswait_state *pvcontrol,
     if (rc < 0)
         return rc;
 
+    if (!rc)
+        return ERROR_NOPARAVIRT;
+
     shutdown_path = libxl__domain_pvcontrol_xspath(gc, domid);
     if (!shutdown_path)
         return ERROR_FAIL;