Not every chardev is plugged onto virtio-serial bus. However, the
code introduced in 89e991a2aa36b04 assumes that. Incorrectly.
With previous patches we have three options where a chardev can
be plugged: virtio-serial, USB and PCI. This commit fixes the
attach part. However, since we are not auto allocating USB
addresses yet, I'm just marking the place where appropriate code
should go.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>