qemuDomainDetachChrDevice: Fix chardev hot-unplug
    
    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
    detach 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>