Do not use virtio-serial port 0 for generic ports

Per the discussion in:

https://bugzilla.redhat.com/show_bug.cgi?id=670394

The port numbering should start from 1, not 0.  We assign maxport + 1,
so start maxport at 0.

src/conf/domain_conf.c

@@ -5328,7 +5328,7 @@ static virDomainDefPtr virDomainDefParseXML(virCapsPtr caps,
 
         if (chr->info.type == VIR_DOMAIN_DEVICE_ADDRESS_TYPE_VIRTIO_SERIAL &&
             chr->info.addr.vioserial.port == 0) {
-            int maxport = -1;
+            int maxport = 0;
             int j;
             for (j = 0 ; j < i ; j++) {
                 virDomainChrDefPtr thischr = def->channels[j];