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>

     1.1 --- a/tools/python/xen/xm/main.py	Wed Feb 24 10:44:30 2010 +0000
     1.2 +++ b/tools/python/xen/xm/main.py	Wed Feb 24 10:46:49 2010 +0000
     1.3 @@ -2122,7 +2122,7 @@ def arg_check_for_resource_list(args, na
     1.4          (options, params) = getopt.gnu_getopt(args, 'l', ['long'])
     1.5      except getopt.GetoptError, opterr:
     1.6          err(opterr)
     1.7 -        sys.exit(1)
     1.8 +        usage(name)
     1.9  
    1.10      for (k, v) in options:
    1.11          if k in ['-l', '--long']: