spapr: Allow boot from vhost-*-scsi backends

spapr: Allow boot from vhost-*-scsi backends

The current implementation of spapr_get_fw_dev_path() doesn't take into
consideration vhost-*-scsi devices. This makes said devices unbootable
on PPC as SLOF is unable to work out the path to scan boot disks.

This makes VMs bootable on spapr when using vhost-*-scsi by implementing
a disk path for VHostSCSICommon (which currently includes both
vhost-user-scsi and vhost-scsi).

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
Signed-off-by: Mike Cui <cui@nutanix.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>