vhost-scsi: expose the TYPE_FW_PATH_PROVIDER interface

In the way, we can make the bootindex property take effect.
At the meanwhile, the firmware path name of vhost-scsi is
"channel@channel/vhost-scsi@target,lun".

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
