I was forced the other day to shutdown a machine via command line and my surprise was to get an error during the execution: VM didn't acknowledge the need to shutdown.
The error is quite convincing and full of useful stuff (NOT) but it's there and you cannot get past it without some extra parameters.
Read on for the solution.

[root@xengz ~]# xe vm-list params=all name-label=gzvirt64 | egrep "name-label|power-state"
                    name-label ( RW): gzvirt64
                   power-state ( RO): running

The machine won't shutdown and brigs this error:

[root@xengz ~]# xe vm-shutdown vm=gzvirt64
VM didn't acknowledge the need to shutdown.

If you are sure this machine must be off, follow what's next:

[root@xengz ~]# xe vm-list params=uuid name-label=gzvrit64
uuid ( RO)    : aa1fca03-d5bb-6b08-a449-7f79c28dd7e8
[root@xengz ~]# xe vm-shutdown uuid=aa1fca03-d5bb-6b08-a449-7f79c28dd7e8 force=true
[root@Xengz ~]# xe vm-list params=all name-label=gzvirt64 | egrep "name-label|power-state"
                    name-label ( RW): gzvirt64
                   power-state ( RO): halted

END!