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Reproduced on qemu-kvm-0.12.1.2-2.160.el6.x86_64 and 
verified on qemu-kvm-0.12.1.2-2.195.el6.x86_64 by following command:
# /usr/libexec/qemu-kvm --help | grep nest

Actual Result:
on qemu-kvm-0.12.1.2-2.160.el6.x86_64:
# /usr/libexec/qemu-kvm --help | grep nest
-enable-nesting enable support for running a VM inside the VM (AMD only)
on qemu-kvm-0.12.1.2-2.195.el6.x86_64:
# /usr/libexec/qemu-kvm --help  | grep nest | wc -l
0

Based on this comment, this issue has been fixed.

    Technical note added. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    New Contents:
The -enable-nested qemu-kvm option was removed, as nested virtualization is not supported.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2011-1531.html