Why are KVM guests failing to boot with XML errors?

Issue

• Virtual machine fail to run with following error message:

[root@xyz qemu]# virsh start abc
error: Failed to start domain abc
error: XML error: missing security model when using multiple labels.

• From libvirtd.log;

2013-10-22 15:59:30.373+0000: 19222: error : qemuAutostartDomain:276 : Failed to autostart VM 'abc': XML error: missing security model when using multiple labels

• virt-manager logs:

Tue, 22 Oct 2013 19:58:26 virt-manager 14476] DEBUG (error:80) dialog message: Error restoring domain: XML error: missing security model when using multiple labels : Error restoring domain: XML error: missing security model when using multiple labels

    Traceback (most recent call last):
      File "/usr/share/virt-manager/virtManager/asyncjob.py", line 44, in cb_wrapper
        callback(asyncjob, *args, **kwargs)
      File "/usr/share/virt-manager/virtManager/asyncjob.py", line 65, in tmpcb
        callback(*args, **kwargs)
      File "/usr/share/virt-manager/virtManager/domain.py", line 1063, in startup
        self._backend.create()
      File "/usr/lib64/python2.6/site-packages/libvirt.py", line 678, in create
        if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
    libvirtError: XML error: missing security model when using multiple labels