Description of problem:
Set per-disk seclabel as below, guest start failed with error "error: unsupported configuration: Unable to find security driver for model (null)"

# virsh edit rhel7
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/libvirt/images/rhel7.2.qcow2'>
        **<seclabel relabel='no'/>**
      </source>
      <target dev='vda' bus='virtio'/>
    </disk>

# virsh start rhel7
error: Failed to start domain rhel7
error: unsupported configuration: Unable to find security driver for model (null)

Version-Release number of selected component:
libvirt-2.0.0-6.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Edit domain xml, set per-disk seclabel without model name.
2. Start guest

Actual results:
Edit domain xml successfully, but guest start failed

Expected results:
In step 1, XML document should failed to validate against schema when editing

Additional info:
Set a model with the seclabel, and start guest, NO "Unable to find security driver for model (null)" error appears.
For example:
      <source file='/var/lib/libvirt/images/rhel7.2.qcow2'>
        <seclabel **model='none'** relabel='no'/>
      </source>