Bugfix: Handle error if VM Generation ID device not present

This was crashing due to NULL-pointer dereference

QMP Test case:
==============

(QEMU) query-vm-generation-id
{"error": {"class": "GenericError", "desc": "VM Generation ID device not
found"}}

HMP Test case:
==============
virsh # qemu-monitor-command --hmp 3 info vm-generation-id
VM Generation ID device not found
Signed-off-by: NBen Warren <ben@skyportsystems.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

hmp.c

@@ -2608,9 +2608,11 @@ void hmp_hotpluggable_cpus(Monitor *mon, const QDict *qdict)
 
 void hmp_info_vm_generation_id(Monitor *mon, const QDict *qdict)
 {
-    GuidInfo *info = qmp_query_vm_generation_id(NULL);
+    Error *err = NULL;
+    GuidInfo *info = qmp_query_vm_generation_id(&err);
     if (info) {
         monitor_printf(mon, "%s\n", info->guid);
     }
+    hmp_handle_error(mon, &err);
     qapi_free_GuidInfo(info);
 }

hw/acpi/vmgenid.c

@@ -248,6 +248,7 @@ GuidInfo *qmp_query_vm_generation_id(Error **errp)
     Object *obj = find_vmgenid_dev();
 
     if (!obj) {
+        error_setg(errp, "VM Generation ID device not found");
         return NULL;
     }
     vms = VMGENID(obj);