Description of problem:
the guest's mac will change after attach a vnet with the option persistent and then restart it.

Version-Release number of selected component (if applicable):
libvirt-0.9.8-1.el6.x86_64
qemu-kvm-0.12.1.2-2.209.el6.x86_64
kernel-2.6.32-220.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Attach a vnet with the options --persistent

a healthy guest named rhel6.

# virsh attach-interface rhel6 network default vnet1 --persistent
# virsh dumpxml rhel6 | grep -3 vnet1
 <interface type='network'>
      <mac address='52:54:00:B5:58:B0'/>
      <source network='default'/>
      <target dev='vnet1'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>


2.Dettach the vnet.

#virsh detach-interface rhel6 network --mac 52:54:00:b5:58:b0 --persistent
error: Failed to detach interface
error: invalid argument: no nic of mac 52:54:00:B5:58:B0


3.Restart the guest and check the mac

# virsh destroy rhel6
Domain rhel6 destroyed
# virsh dumpxml rhel6
/*you will find this*/
 <interface type='network'>
      <mac address='52:54:00:f8:5e:7a'/>
      <source network='default'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>

#virsh start rhel6
Domain rhel6 started

# virsh dumpxml rhel6 | grep -3 vnet1
    <interface type='network'>
      <mac address='52:54:00:f8:5e:7a'/>
      <source network='default'/>
      <target dev='vnet1'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>




Actual results:
the mac change from 52:54:00:B5:58:B0 to 52:54:00:f8:5e:7a

Expected results:
the mac should not change

Additional info:
If no the --persistent , it work well