conf: Format interface's driver more frequently

https://bugzilla.redhat.com/show_bug.cgi?id=1128751

There's this <driver/> element under <interface/> which can have
several attributes. However, the driver element is currently formated
only if the driver's name or txmode has been specified. This makes
only a little sense as we parse even partial <driver/>, for instance:

    <interface type='user'>
      <mac address='52:54:00:e5:48:58'/>
      <model type='virtio'/>
      <driver ioeventfd='on' event_idx='on' queues='5'/>
    </interface>

But such XML would never get formatted back.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/conf/domain_conf.c

@@ -16286,7 +16286,9 @@ virDomainNetDefFormat(virBufferPtr buf,
         virBufferEscapeString(buf, "<model type='%s'/>\n",
                               def->model);
         if (STREQ(def->model, "virtio") &&
-            (def->driver.virtio.name || def->driver.virtio.txmode)) {
+            (def->driver.virtio.name || def->driver.virtio.txmode ||
+             def->driver.virtio.ioeventfd || def->driver.virtio.event_idx ||
+             def->driver.virtio.queues)) {
             virBufferAddLit(buf, "<driver");
             if (def->driver.virtio.name) {
                 virBufferAsprintf(buf, " name='%s'",

tests/qemuxml2argvdata/qemuxml2argv-interface-driver.xml

+<domain type='qemu'>
+  <name>test</name>
+  <uuid>15d091de-0181-456b-9554-e4382dc1f1ab</uuid>
+  <memory unit='KiB'>1048576</memory>
+  <currentMemory unit='KiB'>1048576</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='pc-0.13'>hvm</type>
+    <boot dev='cdrom'/>
+    <boot dev='hd'/>
+    <bootmenu enable='yes'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>restart</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <disk type='file' device='disk'>
+      <driver name='qemu' type='qcow2' event_idx='on'/>
+      <source file='/var/lib/libvirt/images/f14.img'/>
+      <target dev='vda' bus='virtio'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
+    </disk>
+    <disk type='file' device='cdrom'>
+      <driver name='qemu' type='raw'/>
+      <source file='/var/lib/libvirt/Fedora-14-x86_64-Live-KDE.iso'/>
+      <target dev='hdc' bus='ide'/>
+      <readonly/>
+      <address type='drive' controller='0' bus='1' target='0' unit='0'/>
+    </disk>
+    <controller type='usb' index='0'/>
+    <controller type='virtio-serial' index='0'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
+    </controller>
+    <controller type='ide' index='0'/>
+    <controller type='pci' index='0' model='pci-root'/>
+    <interface type='user'>
+      <mac address='52:54:00:e5:48:58'/>
+      <model type='virtio'/>
+      <driver ioeventfd='on' event_idx='on' queues='5'/>
+    </interface>
+    <serial type='pty'>
+      <target port='0'/>
+    </serial>
+    <console type='pty'>
+      <target type='serial' port='0'/>
+    </console>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>

tests/qemuxml2xmltest.c

@@ -299,6 +299,7 @@ mymain(void)
     DO_TEST("lease");
     DO_TEST("event_idx");
     DO_TEST("vhost_queues");
+    DO_TEST("interface-driver");
     DO_TEST("virtio-lun");
 
     DO_TEST("usb-redir");