fix #3608: unbreak removal of scsi controller

fix #3608: unbreak removal of scsi controller

the assumption that the index of the controller matches that of the last
removed drive only holds for virtio-scsi-single controller, which makes
the old code print a warning when removing the last drive of a
non-virtio-scsi-single controller except when the indices line up by
chance.

we can simply only call a simplified qemu_iothread_del when removing a
scsi disk of a VM with the virtio-scsi-single controller, and skip the
call for the other controllers which don't support io-threads anyway.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>