qemu: Fix segfault in qemuDomainPrimeVirtioDeviceAddresses

On aarch64, lauch vm with the follow configuration:

<interface type="hostdev" managed="yes">
  <mac address="fa:16:3e:14:41:00"/>
  <source>
    <address type="pci" domain="0x0000" bus="0x01" slot="0x0b" function="0x2"/>
  </source>
</interface>

libvirtd will crash when accessing net->model.
Signed-off-by: NWang Yechao <wang.yechao255@zte.com.cn>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

src/qemu/qemu_domain_address.c

@@ -324,7 +324,8 @@ qemuDomainPrimeVirtioDeviceAddresses(virDomainDefPtr def,
     for (i = 0; i < def->nnets; i++) {
         virDomainNetDefPtr net = def->nets[i];
 
-        if (STREQ(net->model, "virtio") &&
+        if (net->model &&
+            STREQ(net->model, "virtio") &&
             net->info.type == VIR_DOMAIN_DEVICE_ADDRESS_TYPE_NONE) {
             net->info.type = type;
         }