[libvirt] how to use libvirt hook scripts to modify domain xml while its start

[cokabug]

i want add sound card to domain xml when guest start,after google,i found
libvirt provide hook function.
i follow the instruction to make a python,and boot a guest,but after use
"virsh edit" its still dont have sound card config


/etc/libvirt/hooks/qemu:
#!/usr/bin/python

import sys
import re
import os

hooklog = '/tmp/hook.log'
log = open(hooklog, 'w')
stdinxml = sys.stdin.readlines()

if sys.argv[2] == 'start':
  log.write("hook start,domain name: %s \n" % sys.argv[1])
  maxslot=-1

  for line in stdinxml:
    slotm = re.search("slot='(?P<slotnum>0x[0-9a-fA-F]{2})'", line)
    if slotm:
      slotnum = int(slotm.group('slotnum'),0)
      if slotnum > maxslot:
        maxslot = slotnum
    if '</devices>' in line:
      log.write("insert sound card config \n")
      slotnum = maxslot + 1
      line = "    <sound model='ich6'>\n"
      line = line + "      <address type='pci' domain='0x0000' bus='0x00'
slot='0x%0.2X' function='0x0'/>\n" % slotnum
      line = line + "    </sound>\n"
      line = line + "  </devices>\n"
    stdoutxml = ''.join(line)
    sys.stdout.write(stdoutxml)

tail -f /tmp/hook.log:
hook start,domain name: instance-00000222
insert sound card config

tail /var/log/libvirt/libvirtd.log:
2014-04-16 07:13:39.159+0000: 52199: warning : qemuDomainObjTaint:1377 :
Domain id=81 name='instance-00000222'
uuid=974d62b7-f316-4f20-a91c-d11cb85980fe is tainted: high-privileges


in these log,hook script seems to executed,but not effective.

can anyone tell me where is wrong?
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