domU creat failed using XL on FVP

XEN and dom0 started successfully on FVP, but when creat domU using XL tool, the following errors occur.

libxl:error:libxl_exec.c:118:libxl_report_child_exitstatus:/etc/xen/scripts/block add[1985] exited with error status 127

libxl:error:libxl_creat.c:1298:domcreat_launch_dm:Domain6:unable to add disk devices

please help me analyze it, thanks.