s390x/virtio-ccw: migration and virtio for 2.4

1. Migration fixups
2. virtio 9pfs

# gpg: Signature made Tue Jun  9 09:00:05 2015 BST using RSA key ID B5A61C7C
# gpg: Good signature from "Christian Borntraeger (IBM) <borntraeger@de.ibm.com>"

* remotes/borntraeger/tags/s390x-20150609:
  s390x/migration: add comment about floating point migration
  s390x/kvm: always ignore empty vcpu interrupt state
  virtio-ccw/migration: Migrate config vector for virtio devices
  virtio-ccw: add support for 9pfs

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>