xm: Fix string index out of range in 'vcpu-pin' command

if <PCPUs> contins '' (e.g. '1,,2'), error 'string index out of range'
occurs. Fix this trivial bug.

Signed-off-by: Yu Zhiguo <yuzg@cn.fujitsu.com>
xen-unstable changeset:   21126:aae7cb2f1841
xen-unstable date:        Fri Apr 09 08:54:25 2010 +0100

tools/python/xen/xm/main.py

@@ -1458,6 +1458,8 @@ def xm_vcpu_pin(args):
     def cpu_make_map(cpulist):
         cpus = []
         for c in cpulist.split(','):
+            if c == '':
+                continue
             if c.find('-') != -1:
                 (x,y) = c.split('-')
                 for i in range(int(x),int(y)+1):