Description of problem:
model = 'virtio-scsi' tag is getting in the guest xml when the virtio-disk is attached

Steps to reproduce:

1. Create the file using 
qemu-img create -f qcow2 /home/virtio-scsi1.qcow2 1G

2. Attach the disk 
 virsh attach-disk RHEL_WORKING /home/virtio-scsi1.qcow2 sdd --persistent  --cache writethrough --sourcetype file --driver qemu --subdriver qcow2

3. Check the guest xml the model='virtio-scsi' is missing

<disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='writethrough'/>
      <source file='/home/virtio-scsi1.qcow2'/>
      <target dev='sdd' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='3'/>
</disk>

git log | head
commit 4b744d7d00e18ee7c0d8f89542954743d64ffeef
Author: Michal Privoznik <mprivozn>
Date:   Mon Oct 7 19:21:47 2013 +0200

    virerror: s/VIR_ERR_STORAGE_VOL_EXISTS/VIR_ERR_STORAGE_VOL_EXISTS/
    
    We currently have other error codes in singular form, e.g.
    VIR_ERR_NETWORK_EXIST. Cleanup the previous patch to match the form.
    
    Signed-off-by: Michal Privoznik <mprivozn>

libvirtd --version
libvirtd (libvirt) 1.1.3


Tried to edit the model ='virtio-scsi' manually for <target dev='sdd' bus='scsi'/> its not saving. 

There is patch for it 

http://www.redhat.com/archives/libvir-list/2012-November/msg00565.html