libxl: Properly dispose libxl_domain_config object

V2 of the libxl soft reset patch, which was pushed as commit da4b0fd9,
dropped the hunk that disposed of the libxl_domain_config object. Add
the missing hunk to properly dispose the object.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>

src/libxl/libxl_domain.c

@@ -605,6 +605,7 @@ libxlDomainShutdownThread(void *opaque)
     virObjectEventStateQueue(driver->domainEventState, dom_event);
     libxl_event_free(cfg->ctx, ev);
     VIR_FREE(shutdown_info);
+    libxl_domain_config_dispose(&d_config);
     virObjectUnref(cfg);
 }