parallels: fix usage of disk->info.addr.drive structure

parallels: fix usage of disk->info.addr.drive structure

For SCSI and SATA devices controller and unit are used
to specify drive address. For IDE devices - bus specifies
IDE bus, becase usually there are 2 IDE buses on IDE
controller.

Parallels SDK allows to set drive position by calling
PrlVmDev_SetStackIndex. Since PCS VMs have only one
controller of each type, for SATA and SCSI devices it
simple means position on bus, for IDE devices -
2 * bus_number + position_on_bus.

This patch fixes mapping from libvirt's disk->info.addr.drive
to parallels's 'StackIndex'.

Signed-off-by: Dmitry Guryanov <dguryanov@parallels.com>