Some minor improvements to the with-vdi script
Some minor improvements to the with-vdi script

- return the user command's exit code as the exit code of with-vdi script (unless something else failed in the preparation or post run steps)
- relax the requirement on the user command parameter, both absolute paths and in-paths executables are acceptable
- allow a user command to have its own arguments (i.e. with-vdi vdi-uuid user-command arg1 arg2 ... argn)

Signed-off-by: Zheng Li <34c6fceca75e456f25e7e99531e2425c6c1de443@zheng.li>

djs55/xcp-rrdd,djs55/xcp-networkd,koushikcgit/xcp-networkd,koushikcgit/xcp-rrdd,sharady/xcp-networkd,koushikcgit/xcp-networkd,simonjbeaumont/xcp-rrdd,koushikcgit/xcp-rrdd,djs55/xcp-networkd,johnelse/xcp-rrdd,robhoes/squeezed,sharady/xcp-networkd,djs55/squeezed,djs55/xcp-rrdd,johnelse/xcp-rrdd,simonjbeaumont/xcp-rrdd,koushikcgit/xcp-rrdd