Description of problem:
vhost_net module is not loaded by default and we can boot up a guest successfully.
But, after modprobe it (still can boot up guest at this moment) and then remove it, the guest can not boot up with the following message:

warning: could not open /dev/net/tun: no virtual network emulation
qemu-kvm: -net tap,vlan=0,script=/etc/qemu-ifup,ifname=hello: Device 'tap' could not be initialized

Command line:
# /usr/libexec/qemu-kvm -m 2G -smp 2 -rtc-td-hack -usbdevice tablet -no-kvm-pit-reinjection  -drive file=win2k8-r2.qcow2,media=disk,if=ide,cache=off,werror=stop,rerror=stop -net nic,vlan=0,macaddr=1e:2d:0a:18:9f:4d,model=e1000 -net tap,vlan=0,script=/etc/qemu-ifup,ifname=hello -vnc :10 -monitor stdio -boot c -cpu qemu64,+sse2 -uuid b1428152-97b8-4bd3-a24c-16c91177fa31 -startdate now -balloon none -name 161


Version-Release number of selected component (if applicable):
qemu-kvm-0.12.1.2-2.51.el6.x86_64
kernel: 2.6.32-24.el6.x86_64

How reproducible:
Always

Steps to Reproduce:
1.Boot a guest with the above command line.
2.Add the vhost_net module.
#modprobe  vhost_net
#lsmod | grep vhost_net
3.Boot the guest again.
4.Remove the vhost_net module.
#modprobe -r vhost_net
#lsmod | grep vhost_net
5.Boot the guest again with the same command line.
  
Actual results:
After step 5, guest can not boot up.

warning: could not open /dev/net/tun: no virtual network emulation

Expected results:
Guest can boot up successfully all the time.

Additional info: