(grnet) Enable -device option for old disks upon migration

(grnet) Enable -device option for old disks upon migration

Change _GenerateDeviceKVMId() to take device index as an
extra argument. In case device has uuid and pci info
we return proper device id. In case idx is given we return
a dummy id (e.g. hotdisk-0). Otherwise we raise an Exception.

This is used only by GenerateKVMBlockDevicesOptions() for
block_devices found in runtime files that have not uuid or
pci info (Ganeti 2.6.2 hotplug implementation). By returning
a dummy id we enable -device option in for paravirtual disk types.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>