virDomainDefCompatibleDevice: Relax alias change check

https://bugzilla.redhat.com/show_bug.cgi?id=1621910

When introducing this check back in 4ad54a41 my mindset was
that if an element is missing in update XML then user is
requesting for removal of the corresponding setting. For
instance, if <bandwidth/> is not present in update XML any QoS
previously set on <interface/> is cleared out. Well this
assumption is correct but only to some extent.

Turns out, we have some users who when updating path to ISO
image construct very minimalistic disk XML and pass it to device
update API. Such XML is lacking a lot of information, and alias
is one of them. This triggers error in
virDomainDefCompatibleDevice() because we think that user is
requesting to remove the alias. Well, they are not.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/conf/domain_conf.c

@@ -28785,9 +28785,9 @@ virDomainDefCompatibleDevice(virDomainDefPtr def,
 
     if (action == VIR_DOMAIN_DEVICE_ACTION_UPDATE &&
         live &&
-        ((!!data.newInfo != !!data.oldInfo) ||
-         (data.newInfo && data.oldInfo &&
-          STRNEQ_NULLABLE(data.newInfo->alias, data.oldInfo->alias)))) {
+        (data.newInfo && data.oldInfo &&
+         data.newInfo->alias && data.oldInfo->alias &&
+         STRNEQ(data.newInfo->alias, data.oldInfo->alias))) {
         virReportError(VIR_ERR_OPERATION_DENIED, "%s",
                        _("changing device alias is not allowed"));
         return -1;