xm list: Return unique exit code for non-existent domain

This patch will make xm return a exit code of 3 if `xm list
<non_existant_domain>` is done rather than the generic code of 1.  I
used 3 because XendClient had a macro setup pointing
ERROR_INVALID_DOMAIN to 3.

From: "Shaun R." <mailinglists@unix-scripts.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

tools/python/xen/xm/main.py

@@ -2926,6 +2926,7 @@ def _run_cmd(cmd, cmd_name, args):
     except xmlrpclib.Fault, ex:
         if ex.faultCode == XendClient.ERROR_INVALID_DOMAIN:
             err("Domain '%s' does not exist." % ex.faultString)
+            return False, ex.faultCode
         else:
             err(ex.faultString)
             _usage(cmd_name)