libxl: Report connect type as Xen

Currently, the libxl driver reports a connection type of "xenlight".
To be compatible with the legacy Xen driver, it should return "Xen".

Note: I noticed this while testing the libxl driver on OpenStack.
After switching my Xen compute nodes to use the libxl stack, I
could no longer launch instances on those nodes since
hypervisor_type was reported as "xenlight" instead of "xen".

src/libxl/libxl_driver.c

@@ -1405,7 +1405,7 @@ libxlConnectClose(virConnectPtr conn ATTRIBUTE_UNUSED)
 static const char *
 libxlConnectGetType(virConnectPtr conn ATTRIBUTE_UNUSED)
 {
-    return "xenlight";
+    return "Xen";
 }
 
 static int