virtio: console: make discard_port_data() use get_inbuf()

discard_port_data() used virtqueue_get_buf() directly instead of using
get_inbuf().  Fix this, so that we get accounting for all received
bytes.  This also simplifies the code a lot.

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