qemu_hotplug: Detach guestfwd using netdev_del

qemu_hotplug: Detach guestfwd using netdev_del

https://bugzilla.redhat.com/show_bug.cgi?id=1624204

The guestfwd channels are -netdevs really. Hotunplug them as
such. Also, DEVICE_DELETED event is not triggered (surprisingly,
since we're not issuing device_del rather than netdev_del) and
associated chardev is removed automagically too. This means that
we need to do qemuDomainRemoveChrDevice() minus monitor call to
remove the chardev.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>