bhyve: fix crash on missing interface model

The bhyve driver crashes in bhyveBuildNetArgStr() when
network interface model is not defined. As it has to be provided
explicitly, add a check to report an error if it's missing.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/bhyve/bhyve_command.c

@@ -56,6 +56,12 @@ bhyveBuildNetArgStr(virConnectPtr conn,
     int ret = -1;
     virDomainNetType actualType = virDomainNetGetActualType(net);
 
+    if (net->model == NULL) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("NIC model must be specified"));
+        return -1;
+    }
+
     if (STREQ(net->model, "virtio")) {
         if (VIR_STRDUP(nic_model, "virtio-net") < 0)
             return -1;