Improve libvirt_vm.add_nic to support ovs virtualport attribute

It should be a design flaw; when netdst is ovs, attribute
virtualport_type needs to be added

Signed-off-by: Lei Yang <leiyang@redhat.com>

Author: yanglei-rh

virttest/libvirt_vm.py

@@ -28,10 +28,12 @@
     error_context,
     kernel_interface,
     libvirt_xml,
+    openvswitch,
     storage,
     test_setup,
     utils_logfile,
     utils_misc,
+    utils_net,
     utils_package,
     utils_selinux,
     virsh,
@@ -939,7 +941,12 @@ def add_nic(help_text, nic_params):
             else:
                 # newer libvirt (--network=mynet,model=virtio,mac=00:11)
                 if nettype != "user":
+                    br_name = utils_net.find_bridge_manager(netdst)
+                    if br_name is None:
+                        raise exceptions.TestError(f"Bridge '{netdst}' does not exist")
                     result += "=%s" % netdst
+                    if isinstance(br_name, openvswitch.OpenVSwitchSystem):
+                        result += ",virtualport_type=openvswitch"
                 if nettype and nic_model:  # only supported along with nettype
                     result += ",model=%s" % nic_model
                 if nettype and mac: