Turn virCapabilities into a virObject

To enable virCapabilities instances to be reference counted,
turn it into a virObject. All cases of virCapabilitiesFree
turn into virObjectUnref

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

tests/xmconfigtest.c

@@ -245,7 +245,7 @@ mymain(void)
     DO_TEST("no-source-cdrom", 2);
     DO_TEST("pci-devs", 2);
 
-    virCapabilitiesFree(caps);
+    virObjectUnref(caps);
 
     return ret==0 ? EXIT_SUCCESS : EXIT_FAILURE;
 }