Merge remote-tracking branch 'remotes/cohuck/tags/s390x-20150508' into staging

Assorted s390x patches:
- updates for virtio-ccw and s390-virtio, making them more similar
  to virtio-pci
- improvements regarding per-vcpu interrupts and migration

# gpg: Signature made Fri May  8 09:45:09 2015 BST using RSA key ID C6F02FAF
# gpg: Good signature from "Cornelia Huck <huckc@linux.vnet.ibm.com>"
# gpg:                 aka "Cornelia Huck <cornelia.huck@de.ibm.com>"

* remotes/cohuck/tags/s390x-20150508:
  s390x/kvm: migrate vcpu interrupt state
  s390x: move fpu regs into a subsection of the vmstate
  s390x/kvm: use ioctl KVM_S390_IRQ for vcpu interrupts
  virtio-ccw: implement ->device_plugged
  virtio-ccw: change realization sequence
  s390-virtio: clear {used,avail}_event_idx on reset as well
  s390-virtio: use common features
  s390-virtio: Accommodate guests using virtqueues too early

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