virtio: console: Ignore port name update request if name already set

We don't allow port name changes dynamically for a port.  So any
requests by the host to change the name are ignored.

Before this patch, if the hypervisor sent a port name while we had one
set already, we would leak memory equivalent to the size of the old
name.

This scenario wasn't expected so far, but with the suspend-resume
support, we'll send the VIRTIO_CONSOLE_PORT_READY message after restore,
which can get us into this situation.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>