Does anyone remember why the the vbd frontend drivers also claim the
SCSI disks, but the vbd backend in qemu has no unplug support for SCSI?

The current situation for qemu-xen and qemu-xen-traditional is that both
will create an emulated LSI controller with disk=[vdev=sda]. The
xenlinux and pvops frontend drivers will claim the disk, but the
sym53c8xx will see and claim it as well. As a result each disk is
visible twice. One has to blacklist the sym53c8xx driver to avoid that.

What should be done to fix this?
#1 new unplug protocol for SCSI, but old guests dont know about it
#2 reuse IDE flag to also unplug SCSI. This would cover pvops and guests
   where xenlinux based xen-platform-pci.ko is loaded before
   sym53c8xx.ko. It would break guests with frontend drivers that do not
   claim SCSI disks, the SCSI disk would disappear (if such frontends
   really exist).
#3 do not provide SCSI if guest has PV drivers

Olaf