The legacy host registration/unregistration is the last user
of the scsi_host_list but it really wants a per-template list
instead, so switch to one that is maintained in
scsi_register/scsi_unregister.  Also the legacy init/exit code
is small enough now to be self-contained in scsi_module.c now.

This second version also has proper failure handling in the
init_this_scsi_driver