qdev-ify virtio-blk.

First user of the new drive property.  With this patch applied host
and guest config can be specified separately, like this:

  -drive if=none,id=disk1,file=/path/to/disk.img
  -device virtio-blk-pci,drive=disk1

You can set any property for virtio-blk-pci now.  You can set the pci
address via addr=.  You can switch the device into 0.10 compat mode
using class=0x0180.  As this is per device you can have one 0.10 and one
0.11 virtio block device in a single virtual machine.

Old syntax continues to work.  Internally it does the same as the two
lines above though.  One side effect this has is a different
initialization order, which might result in a different pci address
being assigned by default.

Long term plan here is to have this working for all block devices, i.e.
once all scsi is properly qdev-ified you will be able to do something
like this:

  -drive if=none,id=sda,file=/path/to/disk.img
  -device lsi,id=lsi,addr=<pciaddr>
  -device scsi-disk,drive=sda,bus=lsi.0,lun=<n>

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
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