[S390] Add hotplug support

I just submitted a few patches that enable the s390 virtio bus to receive
a hotplug add event. This patch implements the qemu side of it, so that new
hotplug events can be submitted to the guest.

Signed-off-by: Alexander Graf <agraf@suse.de>

---

v1 -> v2:

  - make s390 virtio hoplug code emulate-capable