"usb-hc-create" does not check usb-ver parameter. It allows 2/2.0/2.0usb/2.0aaa. While low level 
driver doing hc create, it gets an integer by vssanf %d from usb-ver string, so there is no problem. 
But 2/2.0/2.0usb/2.0aaa will be saved into VM config.

After that, while doing "usb-list", it cannot handle "2.0/2.0usb/2.0aaa" and will cause error:
Idx BE  state usb-ver  BE-path
Error: Invalid argument.
Usage: xm usb-list <Domain>

This patch is to let "usb-list" handle all usb-ver cases as low level driver does and won't cause error.

About this problem, I've submitted two patches to upstream before, but got no response. Information
could be referred to: 
    http://www.gossamer-threads.com/lists/xen/devel/178406?search_string=usb-list;#178406
    http://www.gossamer-threads.com/lists/xen/devel/181021?search_string=usb-list;#181021


Index: xen-4.1.0-testing/tools/python/xen/xm/main.py
===================================================================
--- xen-4.1.0-testing.orig/tools/python/xen/xm/main.py
+++ xen-4.1.0-testing/tools/python/xen/xm/main.py
@@ -2631,10 +2631,22 @@ def xm_usb_list(args):
         ni = parse_dev_info(x[1])
         ni['idx'] = int(x[0])
         usbver = sxp.child_value(x[1], 'usb-ver')
+
+        substr = re.search("^\d{1,}", usbver)
+        if substr:
+            usbver = substr.group()
+        else:
+            print "Unknown usb-ver"
+            continue
+
         if int(usbver) == 1:
             ni['usb-ver'] = 'USB1.1'
-        else:
+        elif int(usbver) == 2:
             ni['usb-ver'] = 'USB2.0'
+        else:
+            print "Unknown usb-ver"
+            continue
+
         print "%(idx)-3d %(backend-id)-3d %(state)-5d %(usb-ver)-7s %(be-path)-30s  " % ni
 
         ports = sxp.child(x[1], 'port')