xm: Show usage message of xm commands

xm: Show usage message of xm commands

The following commands don't show an usage message even if we give
a wrong option to the commands.

 - xm block-list
 - xm network-list
 - xm network2-list
 - xm vtpm-list
 - xm pci-list
 - xm scsi-list

e.g.
 # xm block-list --xxx vm1
 Error: option --xxx not recognized

This patch shows the usage message of the commands.

e.g.
 # xm block-list --xxx vm1
 Error: option --xxx not recognized
 Usage: xm block-list <Domain> [--long]
 List virtual block devices for a domain.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>