Description of problem:
libvirt didn't check dev value in network xml

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:
everytimes

Steps to Reproduce:
1. prepare the following network xml:
# cat test.xml
<network>
<name>test</name>
<uuid>f33ccec8-b0fa-ca47-229f-0daf225b9eb7</uuid>
<forward dev='*' mode='nat'>
     <interface dev='*'/>
   </forward>
   <bridge name='test' stp='on' delay='0' />
   <mac address='52:54:00:B1:79:5F'/>
   <ip address='192.168.200.1' netmask='255.255.255.0'>
     <dhcp>
       <range start='192.168.200.2' end='192.168.200.254' />
     </dhcp>
   </ip>
 </network>
2. define and start test network
# virsh net-define test.xml 
Network test defined from tmp.xml
# virsh net-start test
Network test started
3. start the guest with test network:
# virsh dumpxml rhel6
...
<interface type='network'>
      <source network='test'/>
      <model type='virtio'/>
      <driver name='qemu'/>function='0x0'/>
    </interface>
...
# virsh start rhel6  
Domain rhel6 started

Actual results:
libvirt didn't check dev value in network xml

Expected results:
libvirt should check dev value in network xml and raise error when start the network

Additional info: