xen vm cannot found harddisk's driver

Hi, everyone:
i'am new to xen. and when i tried to install a centos5 vm using xen, the vm cannot find the harddisk's driver.
the attachment is some pictures of the installation process.

 i used centos5.5 and 'dd' to create the vm's virtual disk image, and my creating command is:
virt-install -n centos5 -r 512 --vcpus=2 --disk path=/vm/centos5.img,bus=scsi --os-type=linux --os-variant=rhel5.4 -l /media/CentOS_5.5_Final
  for the 'bus' option, i had tried all ones in the manual, but none worked.
  
  Could anyone help me about this please? thanks!