qemu: Update balloon info only if job is allowed

In qemuDomainUpdateCurrentMemorySize I misplaced the actual update of
the balloon size to a place where it may not be initialized. Move it a
few lines above.

src/qemu/qemu_domain.c

@@ -3256,9 +3256,9 @@ qemuDomainUpdateCurrentMemorySize(virQEMUDriverPtr driver,
 
         if (ret < 0)
             return -1;
-    }
 
-    vm->def->mem.cur_balloon = balloon;
+        vm->def->mem.cur_balloon = balloon;
+    }
 
     return 0;
 }