libxl: advertise support for migration V3

The libxl driver has long supported migration V3 but has never
indicated so in the connectSupportsFeature API. As a result, apps
such as virt-manager that use the more generic virDomainMigrate API
fail with

libvirtError: this function is not supported by the connection driver:
virDomainMigrate

Add VIR_DRV_FEATURE_MIGRATION_V3 to the list of features marked as
supported in the connectSupportsFeature API.

src/libxl/libxl_driver.c

@@ -5677,6 +5677,7 @@ libxlConnectSupportsFeature(virConnectPtr conn, int feature)
         return -1;
 
     switch (feature) {
+    case VIR_DRV_FEATURE_MIGRATION_V3:
     case VIR_DRV_FEATURE_TYPED_PARAM_STRING:
     case VIR_DRV_FEATURE_MIGRATION_PARAMS:
     case VIR_DRV_FEATURE_MIGRATION_P2P: