virtio: sync the dataplane vring state to the virtqueue before virtio_save

When creating snapshot with the dataplane enabled, the snapshot file gets
not the actual state of virtqueue, because the current state is stored in
VirtIOBlockDataPlane. Therefore, before saving snapshot need to sync
the dataplane vring state to the virtqueue. The dataplane will resume its
work at the next notify virtqueue.

When snapshot loads with loadvm we get a message:
VQ 0 size 0x80 Guest index 0x15f5 inconsistent with Host index 0x0:
    delta 0x15f5
error while loading state for instance 0x0 of device
    '0000:00:08.0/virtio-blk'
Error -1 while loading VM state

to reproduce the error I used the following hmp commands:
savevm snap1
loadvm snap1

qemu parameters:
--enable-kvm -smp 4 -m 1024 -drive file=/var/lib/libvirt/images/centos6.4.qcow2,if=none,id=drive-virtio-disk0,format=qcow2,cache=none,aio=native -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x8,drive=drive-virtio-disk0,id=virtio-disk0 -set device.virtio-disk0.x-data-plane=on

Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Message-id: 1445859777-2982-1-git-send-email-den@openvz.org
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 10a06fd6)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>