Except the scsi host device's controller is "lsilogic", mapping
between the libvirt attributes and scsi-generic properties is:

  libvirt     qemu
-----------------------------------------
  controller  bus ($libvirt_controller.0)
  bus         channel
  target      scsi-id
  unit        lun

For scsi host device with "lsilogic" controller, the mapping is:
('target (libvirt)' must be 0, as it's not used; 'unit (libvirt)
must <= 7).

  libvirt            qemu
----------------------------------------------------------
  controller && bus  bus ($libvirt_controller.$libvirt_bus)
  unit               scsi-id

It's not good to hardcode/hard-check limits of these attributes,
and even worse, these limits are not documented, one has to find
out by either testing or reading the qemu code, I'm looking forward
to qemu expose limits like these one day). For example, exposing
"max_target", "max_lun" for megasas:

static const struct SCSIBusInfo megasas_scsi_info = {
    .tcq = true,
    .max_target = MFI_MAX_LD,
    .max_lun = 255,

    .transfer_data = megasas_xfer_complete,
    .get_sg_list = megasas_get_sg_list,
    .complete = megasas_command_complete,
    .cancel = megasas_command_cancel,
};

Example of the qemu command line (lsilogic controller):

  -drive file=/dev/sg2,if=none,id=drive-hostdev-scsi_host7-0-0-0 \
  -device scsi-generic,bus=scsi0.0,scsi-id=8,\
  drive=drive-hostdev-scsi_host7-0-0-0,id=hostdev-scsi_host7-0-0-0

Example of the qemu command line (virtio-scsi controller):

  -drive file=/dev/sg2,if=none,id=drive-hostdev-scsi_host7-0-0-0 \
  -device scsi-generic,bus=scsi0.0,channel=0,scsi-id=128,lun=128,\
  drive=drive-hostdev-scsi_host7-0-0-0,id=hostdev-scsi_host7-0-0-0
Signed-off-by: NHan Cheng <hanc.fnst@cn.fujitsu.com>
Signed-off-by: NOsier Yang <jyang@redhat.com>