qemu: fix default spice password setting

Set spice password even if default VNC password hasn't been set.

https://bugzilla.redhat.com/show_bug.cgi?id=953720
(cherry picked from commit 4327df7e)

src/qemu/qemu_hotplug.c

@@ -2751,7 +2751,7 @@ qemuDomainChangeGraphicsPasswords(virQEMUDriverPtr driver,
     int ret = -1;
     virQEMUDriverConfigPtr cfg = virQEMUDriverGetConfig(driver);
 
-    if (!auth->passwd && !cfg->vncPassword) {
+    if (!auth->passwd && !defaultPasswd) {
         ret = 0;
         goto cleanup;
     }