This patch is against lk 2.5.46-bk3 which includes
Christoph's work removing the requirement for
detect() functions in LLDDs. He sent me an example
for scsi_debug which I have built on with this patch.

As threatened, this version includes a "scsi_debug_add_host"
parameter. At kernel or module load time this is an absolute
number (0..127 are allowable and 1 is the default). So:
     modprobe scsi_debug scsi_debug_num_devs=20 scsi_debug_add_host=0
will result in no scsi_debug hosts (thus no devices) but
the driver has 20 slots available for devices.

Then a host can be introduced (simulated hotplug) by
     echo 1 > /sysfs/bus/scsi/drivers/scsi_debug/add_host
This causes a scsi_debug host to appear and devices get
found on it [14 in my system: 7 targets (0..6) each with
2 luns (0..1)]. Another host hotplug can be simulated by
     echo 1 > /sysfs/bus/scsi/drivers/scsi_debug/add_host
which results in another 6 devices being attached (for a
total of 20 scsi_debug devices as dictated by the original
scsi_debug_num_devs).

That last (second) scsi_debug host can be removed by
     echo -1 > /sysfs/bus/scsi/drivers/scsi_debug/add_host
and its 6 devices go. Another application of this "echo"
removes the first host and all remaining devices.

Seems to work fine.