Implement dispose method for libxlDomainObjPrivate

When creating a timer/event handler reference counting is used. So it could
be possible (in theory) that libxlDomainObjPrivateFree is called with
reference counting >1. The problem is that libxlDomainObjPrivateFree leave
the object in an invalid state with ctx freed (but still having dandling
pointer). This can lead timer/event handler to core.

This patch implements a dispose method for libxlDomainObjPrivate, and moves
freeing the libxl ctx to the dispose method, ensuring the ctx is valid while
the object's reference count is > 0.
Signed-off-by: NFrediano Ziglio <frediano.ziglio@citrix.com>

src/libxl/libxl_driver.c

@@ -110,6 +110,9 @@ static int
 libxlVmStart(libxlDriverPrivatePtr driver, virDomainObjPtr vm,
              bool start_paused, int restore_fd);
 
+static void
+libxlDomainObjPrivateDispose(void *obj);
+
 /* Function definitions */
 static int
 libxlDomainObjPrivateOnceInit(void)
@@ -117,7 +120,7 @@ libxlDomainObjPrivateOnceInit(void)
     if (!(libxlDomainObjPrivateClass = virClassNew(virClassForObjectLockable(),
                                                    "libxlDomainObjPrivate",
                                                    sizeof(libxlDomainObjPrivate),
-                                                   NULL)))
+                                                   libxlDomainObjPrivateDispose)))
         return -1;
 
     return 0;
@@ -418,14 +421,26 @@ libxlDomainObjPrivateAlloc(void)
 }
 
 static void
-libxlDomainObjPrivateFree(void *data)
+libxlDomainObjPrivateDispose(void *obj)
 {
-    libxlDomainObjPrivatePtr priv = data;
+    libxlDomainObjPrivatePtr priv = obj;
 
     if (priv->deathW)
         libxl_evdisable_domain_death(priv->ctx, priv->deathW);
 
     libxl_ctx_free(priv->ctx);
+}
+
+static void
+libxlDomainObjPrivateFree(void *data)
+{
+    libxlDomainObjPrivatePtr priv = data;
+
+    if (priv->deathW) {
+        libxl_evdisable_domain_death(priv->ctx, priv->deathW);
+        priv->deathW = NULL;
+    }
+
     virObjectUnref(priv);
 }