avoid setting lun number for drives when pvscsi controller is used

avoid setting lun number for drives when pvscsi controller is used

Reported in the community forum[0].

In QEMU's hw/scsi/vmw_pvscsi.c in the SCSIBusInfo struct, the max_lun property
is set to 0. This means that in our stack, one cannot have multiple disks and
use 'scsihw: pvscsi' currently, as kvm would fail with
    bad scsi device lun: 1

Instead of increasing the lun number, increase the scsi-id, as we already do for
lsi.* (in hw/scsi/lsi53c895a.c the max_lun property is also 0).

[0]: https://forum.proxmox.com/threads/kvm-bad-scsi-device-lun-1.84318/

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
Reviewed-by: Stefan Reiter <s.reiter@proxmox.com>
Tested-by: Stefan Reiter <s.reiter@proxmox.com>