lib: Introduce event for tracking disk backing file write threshold

When using thin provisioning, management tools need to resize the disk
in certain cases. To avoid having them to poll disk usage introduce an
event which will be fired when a given offset of the storage is written
by the hypervisor. Together with the API which will be added later, it
will allow registering thresholds for given storage backing volumes and
this event will then notify management if the threshold is exceeded.

tools/virsh-domain.c

@@ -12877,6 +12877,25 @@ virshEventMetadataChangePrint(virConnectPtr conn ATTRIBUTE_UNUSED,
 }
 
 
+static void
+virshEventBlockThresholdPrint(virConnectPtr conn ATTRIBUTE_UNUSED,
+                              virDomainPtr dom,
+                              const char *dev,
+                              const char *path,
+                              unsigned long long threshold,
+                              unsigned long long excess,
+                              void *opaque)
+{
+    virBuffer buf = VIR_BUFFER_INITIALIZER;
+
+    virBufferAsprintf(&buf, _("event 'block-threshold' for domain %s: "
+                              "dev: %s(%s) %llu %llu\n"),
+                      virDomainGetName(dom),
+                      dev, NULLSTR(path), threshold, excess);
+    virshEventPrint(opaque, &buf);
+}
+
+
 static vshEventCallback vshEventCallbacks[] = {
     { "lifecycle",
       VIR_DOMAIN_EVENT_CALLBACK(virshEventLifecyclePrint), },
@@ -12924,6 +12943,8 @@ static vshEventCallback vshEventCallbacks[] = {
       VIR_DOMAIN_EVENT_CALLBACK(virshEventDeviceRemovalFailedPrint), },
     { "metadata-change",
       VIR_DOMAIN_EVENT_CALLBACK(virshEventMetadataChangePrint), },
+    { "block-threshold",
+      VIR_DOMAIN_EVENT_CALLBACK(virshEventBlockThresholdPrint), },
 };
 verify(VIR_DOMAIN_EVENT_ID_LAST == ARRAY_CARDINALITY(vshEventCallbacks));