scsi: push lun field to SCSIDevice

This will let SCSIBus detect requests sent to an invalid LUN, and
handle them itself.  However, there will be still support for only one
LUN per target

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
