cpu_conf: Format vendor_id for host-model CPUs

In commit v5.9.0-400-gaf8e3992 I removed printing model's fallback and
vendor_id attributes when no model is specified. However, vendor_id
makes sense even without a specific CPU model (for host-model CPUs).

https://bugzilla.redhat.com/show_bug.cgi?id=1804549Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/conf/cpu_conf.c

@@ -791,10 +791,10 @@ virCPUDefFormatBuf(virBufferPtr buf,
         return -1;
     }
 
-    if (formatModel && def->model) {
+    if (formatModel && (def->model || def->vendor_id)) {
         virBufferAddLit(buf, "<model");
 
-        if (def->type == VIR_CPU_TYPE_GUEST) {
+        if (def->type == VIR_CPU_TYPE_GUEST && def->model) {
             const char *fallback;
 
             fallback = virCPUFallbackTypeToString(def->fallback);
@@ -805,11 +805,15 @@ virCPUDefFormatBuf(virBufferPtr buf,
                 return -1;
             }
             virBufferAsprintf(buf, " fallback='%s'", fallback);
-            if (def->vendor_id)
-                virBufferEscapeString(buf, " vendor_id='%s'", def->vendor_id);
         }
 
+        if (def->type == VIR_CPU_TYPE_GUEST)
+            virBufferEscapeString(buf, " vendor_id='%s'", def->vendor_id);
+
+        if (def->model)
             virBufferEscapeString(buf, ">%s</model>\n", def->model);
+        else
+            virBufferAddLit(buf, "/>\n");
     }
 
     if (formatModel && def->vendor)

tests/qemuxml2xmloutdata/cpu-host-model-vendor.xml

@@ -8,7 +8,9 @@
     <type arch='x86_64' machine='pc'>hvm</type>
     <boot dev='network'/>
   </os>
-  <cpu mode='host-model' check='partial'/>
+  <cpu mode='host-model' check='partial'>
+    <model vendor_id='Libvirt QEMU'/>
+  </cpu>
   <clock offset='utc'/>
   <on_poweroff>destroy</on_poweroff>
   <on_reboot>restart</on_reboot>