include: libvirt-domain: Add 'message' @reason of VIR_DOMAIN_EVENT_ID_IO_ERROR_REASON

In case when the hypervisor does report the reason for the I/O error as
an unstable string to display to users we can add a @reason possibility
for the I/O error event noting that the error is available.

Add 'message' as a reason enumeration value and document it
to instruct users to look at the logs or virDomainGetMessages().

The resulting event looks like:

 event 'io-error' for domain 'cd': /dev/mapper/errdev0 (virtio-disk0) report due to message

Users then can look at the virDomainGetMessages() API:

 I/O error: disk='vda', index='1', path='/dev/mapper/errdev0', timestamp='2025-01-28 15:47:52.776+0000', message='Input/output error'

Or in the VM log file:

 2025-01-28 15:47:52.776+0000: IO error device='virtio-disk0' node-name='libvirt-1-storage' reason='Input/output error'

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

include/libvirt/libvirt-domain.h

@@ -4812,6 +4812,10 @@ typedef void (*virConnectDomainEventIOErrorCallback)(virConnectPtr conn,
  * - "enospc": The I/O error is known to be caused by an ENOSPC condition in
  *             the host. Resizing the disk source to be larger will allow the
  *             guest to be resumed as if nothing happened.
+ * - "message": The hypervisor reported a string description of the
+ *              I/O error. The errors are usually logged into the
+ *              domain log file or the last instance of the error
+ *              string can be queried via virDomainGetMessages().
  *
  * Since: 0.8.1
  */

src/qemu/qemu_process.c

@@ -862,6 +862,8 @@ qemuProcessHandleIOError(qemuMonitor *mon G_GNUC_UNUSED,
 
     if (nospace)
         eventReason = "enospc";
+    else if (reason)
+        eventReason = "message";
 
     ioErrorEvent = virDomainEventIOErrorNewFromObj(vm, eventPath, eventAlias, action);
     ioErrorEvent2 = virDomainEventIOErrorReasonNewFromObj(vm, eventPath, eventAlias, action, eventReason);