qemu: fix error propagation in qemuMigrationBegin

If virCloseCallbacksSet fails, qemuMigrationBegin must return NULL to
indicate an error occurred.
Signed-off-by: NMichael Chapman <mike@very.puzzling.org>
(cherry picked from commit 77ddd0bb)

src/qemu/qemu_migration.c

@@ -2745,8 +2745,10 @@ qemuMigrationBegin(virConnectPtr conn,
          * place.
          */
         if (virCloseCallbacksSet(driver->closeCallbacks, vm, conn,
-                                 qemuMigrationCleanup) < 0)
+                                 qemuMigrationCleanup) < 0) {
+            VIR_FREE(xml);
             goto endjob;
+        }
         qemuMigrationJobContinue(vm);
     } else {
         goto endjob;