qemu_command: Resolve Coverity REVERSE_INULL

In qemuNetworkIfaceConnect() a call to virNetDevBandwidthSet() is
made where the function prototype requires the first parameter
(net->ifname) to be non NULL.  Coverity complains that the subsequent
non NULL check for net->ifname prior to the next call gets flagged as
an unnecessary check.  Resolve by removing the extra check

src/qemu/qemu_command.c

@@ -370,7 +370,7 @@ qemuNetworkIfaceConnect(virDomainDefPtr def,
                               false) < 0)
         goto cleanup;
 
-    if (net->filter && net->ifname &&
+    if (net->filter &&
         virDomainConfNWFilterInstantiate(conn, def->uuid, net) < 0) {
         goto cleanup;
     }