virsh: display if ZFS storage backend is enabled

Make 'virsh -V' list ZFS storage backend if it's enabled.

tools/virsh.c

@@ -631,6 +631,9 @@ virshShowVersion(vshControl *ctl ATTRIBUTE_UNUSED)
 #endif
 #ifdef WITH_STORAGE_GLUSTER
     vshPrint(ctl, " Gluster");
+#endif
+#ifdef WITH_STORAGE_ZFS
+    vshPrint(ctl, " ZFS");
 #endif
     vshPrint(ctl, "\n");