libxl: Fix possible object refcnt issue

When libxlDomainMigrationDstPrepare adds the @args to an
virNetSocketAddIOCallback using libxlMigrateDstReceive as
the target of the virNetSocketIOFunc @func with the knowledge
that the libxlMigrateDstReceive will virObjectUnref @args
at the end thus not needing to Unref during normal processing
for libxlDomainMigrationDstPrepare.

However, Coverity believes there's an issue with this. The
problem is there can be @nsocks virNetSocketAddIOCallback's
added, but only one virObjectUnref. That means the first
one done will Unref and the subsequent callers may not get
the @args (or @opaque) as they expected. If there's only
one socket returned from virNetSocketNewListenTCP, then sure
that works. However, if it returned more than one there's
going to be a problem.

To resolve this, since we start with 1 reference from the
virObjectNew for @args, we will add 1 reference for each
time @args is used for virNetSocketAddIOCallback. Then
since libxlDomainMigrationDstPrepare would be done with
@args, move it's virObjectUnref from the error: label to
the done: label (since error: falls through). That way
once the last IOCallback is done, then @args will be freed.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
ACKed-by: NMichal Privoznik <mprivozn@redhat.com>

src/libxl/libxl_migration.c

@@ -793,7 +793,7 @@ libxlDomainMigrationDstPrepare(virConnectPtr dconn,
         if (virNetSocketAddIOCallback(socks[i],
                                       VIR_EVENT_HANDLE_READABLE,
                                       libxlMigrateDstReceive,
-                                      args,
+                                      virObjectRef(args),
                                       NULL) < 0)
             continue;
 
@@ -815,7 +815,6 @@ libxlDomainMigrationDstPrepare(virConnectPtr dconn,
         virObjectUnref(socks[i]);
     }
     VIR_FREE(socks);
-    virObjectUnref(args);
     if (priv) {
         virPortAllocatorRelease(priv->migrationPort);
         priv->migrationPort = 0;
@@ -831,6 +830,7 @@ libxlDomainMigrationDstPrepare(virConnectPtr dconn,
         VIR_FREE(hostname);
     else
         virURIFree(uri);
+    virObjectUnref(args);
     virDomainObjEndAPI(&vm);
     virObjectUnref(cfg);
     return ret;