Port 0 is reserved for virtconsole devices for backward compatibility
with the old -virtioconsole (from qemu 0.12) device type.

libvirt prior to commit 8e28c5d40200b4c5d483bd585d237b9d870372e5 used
port 0 for generic ports.  libvirt will no longer do that, but disallow
instantiating generic ports at id 0 from qemu as well.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>