Fix adding ports to OVS bridges without VLAN tags

The introduction of the new VLAN code, along with the fix
from 5e465df6, caused the
addition of OVS ports to fail with the following message:

ovs-vsctl: 00002|vsctl|ERR|: missing column name

This fix takes into account the VLAN arguments are optional,
and correctly sets up the command line to run the "ovs-vsctl"
command to add ports to the OVS bridge.
Signed-off-by: NKyle Mestery <kmestery@cisco.com>
CC: Eric Blake <eblake@redhat.com>

src/util/virnetdevopenvswitch.c

@@ -104,9 +104,15 @@ int virNetDevOpenvswitchAddPort(const char *brname, const char *ifname,
     }
 
     cmd = virCommandNew(OVSVSCTL);
-    if (ovsport->profileID[0] == '\0') {
+
     virCommandAddArgList(cmd, "--", "--may-exist", "add-port",
-                        brname, ifname, virBufferCurrentContent(&buf),
+                        brname, ifname, NULL);
+
+    if (virBufferUse(&buf) != 0)
+        virCommandAddArgList(cmd, virBufferCurrentContent(&buf), NULL);
+
+    if (ovsport->profileID[0] == '\0') {
+        virCommandAddArgList(cmd,
                         "--", "set", "Interface", ifname, attachedmac_ex_id,
                         "--", "set", "Interface", ifname, ifaceid_ex_id,
                         "--", "set", "Interface", ifname, vmid_ex_id,
@@ -114,8 +120,7 @@ int virNetDevOpenvswitchAddPort(const char *brname, const char *ifname,
                         "external-ids:iface-status=active",
                         NULL);
     } else {
-        virCommandAddArgList(cmd, "--", "--may-exist", "add-port",
-                        brname, ifname, virBufferCurrentContent(&buf),
+        virCommandAddArgList(cmd,
                         "--", "set", "Interface", ifname, attachedmac_ex_id,
                         "--", "set", "Interface", ifname, ifaceid_ex_id,
                         "--", "set", "Interface", ifname, vmid_ex_id,