qemu_hotplug: Support interface type of vhost-user hotplug

qemu_hotplug: Support interface type of vhost-user hotplug

https://bugzilla.redhat.com/show_bug.cgi?id=1366108

There are couple of things that needs to be done in order to
allow vhost-user hotplug. Firstly, vhost-user requires a chardev
which is connected to vhost-user bridge and through which qemu
communicates with the bridge (no acutal guest traffic is sent
through there, just some metadata). In order to generate proper
chardev alias, we must assign device alias way sooner.

Then, because we are plugging the chardev first, we need to do
the proper undo if something fails - that is remove netdev too.
We don't want anything to be left over in case attach fails at
some point.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>