qemu: Resolve Coverity FORWARD_NULL

Coverity notices that net->ifname is potentially referenced after a
VIR_FREE().  Since the net->ifname will eventually be free'd during
virDomainDefFree when calling virDomainNetDefFree, let's just that
processing take care the free.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_process.c

@@ -5333,7 +5333,6 @@ void qemuProcessStop(virQEMUDriverPtr driver,
                              virDomainNetGetActualDirectMode(net),
                              virDomainNetGetActualVirtPortProfile(net),
                              cfg->stateDir));
-            VIR_FREE(net->ifname);
             break;
         case VIR_DOMAIN_NET_TYPE_BRIDGE:
         case VIR_DOMAIN_NET_TYPE_NETWORK: