Merge remote-tracking branch 'bonzini/virtio' into staging

# By Andreas Färber (18) and Paolo Bonzini (12)
# Via Paolo Bonzini
* bonzini/virtio: (30 commits)
  virtio: Convert exit to unrealize
  virtio: Complete converting VirtioDevice to QOM realize
  virtio-scsi: Convert to QOM realize
  virtio-rng: Convert to QOM realize
  virtio-balloon: Convert to QOM realize
  virtio-net: Convert to QOM realize
  virtio-serial: Convert to QOM realize
  virtio-blk: Convert to QOM realize
  virtio-9p: Convert to QOM realize
  virtio: Start converting VirtioDevice to QOM realize
  virtio-scsi: QOM realize preparations
  virtio-rng: QOM realize preparations
  virtio-balloon: QOM realize preparations
  virtio-net: QOM realize preparations
  virtio-serial: QOM realize preparations
  virtio-blk: QOM realize preparations
  virtio-9p: QOM realize preparations
  virtio-blk-dataplane: Improve error reporting
  virtio-pci: add device_unplugged callback
  virtio-rng: switch exit callback to VirtioDeviceClass
  ...