Verified with the following pkgs:

libvirt-0.9.10-6.el6
qemu-kvm-0.12.1.2-2.248.el6
# uname -r
2.6.32-251.el6.x86_64


Steps:
1. Check definition for this cpu model:

# cat /usr/share/libvirt/cpu_map.xml
...
  <model name='Opteron_G4'>
      <vendor name='AMD'/>
      <feature name='fpu'/>
      <feature name='de'/>
      <feature name='pse'/>
      <feature name='tsc'/>
      <feature name='msr'/>
      <feature name='pae'/>
      <feature name='mce'/>
      <feature name='cx8'/>
      <feature name='apic'/>
      <feature name='sep'/>
      <feature name='mtrr'/>
      <feature name='pge'/>
      <feature name='mca'/>
      <feature name='cmov'/>
      <feature name='pat'/>
      <feature name='pse36'/>
      <feature name='clflush'/>
      <feature name='mmx'/>
      <feature name='fxsr'/>
      <feature name='sse'/>
      <feature name='sse2'/>
      <feature name='pni'/>
      <feature name='pclmuldq'/>
      <feature name='ssse3'/>
      <feature name='cx16'/>
      <feature name='sse4.1'/>
      <feature name='sse4.2'/>
      <feature name='popcnt'/>
      <feature name='aes'/>
      <feature name='xsave'/>
      <feature name='avx'/>
      <feature name='syscall'/>
      <feature name='nx'/>
      <feature name='pdpe1gb'/>
      <feature name='rdtscp'/>
      <feature name='lm'/>
      <feature name='lahf_lm'/>
      <feature name='svm'/>
      <feature name='abm'/>
      <feature name='sse4a'/>
      <feature name='sse4a'/>
      <feature name='misalignsse'/>
      <feature name='3dnowprefetch'/>
      <feature name='xop'/>
      <feature name='fma4'/>
    </model>

2. Tested with machine in beaker: amd-dinar-07.lab.bos.redhat.com, check the cpu model should be Opteron_G4.

# virsh capabilities
<capabilities>

  <host>
    <uuid>00001111-0000-2222-0000-888800009999</uuid>
    <cpu>
      <arch>x86_64</arch>
      <model>Opteron_G4</model>  
      <vendor>AMD</vendor>
      <topology sockets='2' cores='8' threads='2'/>
      <feature name='nodeid_msr'/>
      <feature name='wdt'/>
      <feature name='skinit'/>
      <feature name='ibs'/>
      <feature name='osvw'/>
      <feature name='cr8legacy'/>
      <feature name='extapic'/>
      <feature name='cmp_legacy'/>
      <feature name='fxsr_opt'/>
      <feature name='mmxext'/>
      <feature name='osxsave'/>
      <feature name='monitor'/>
      <feature name='ht'/>
      <feature name='vme'/>
    </cpu>
...

    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:
Cause
    AMD Bulldozer CPU was not supported
Consequence
    Nobody could make any use of this processor nor it's flags
Change
    The description for AMD Bulldozer CPU was added
Result
    Everyone can make the use of this processor and it's flags

    Technical note updated. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    Diffed Contents:
@@ -1,8 +1 @@
-Cause
+In Red Hat Enterprise Linux 6.3, libvirt has been updated to add support family 15h microarchitecture AMD processors. With this update, libvirt now leverages new features these processors include.-    AMD Bulldozer CPU was not supported
-Consequence
-    Nobody could make any use of this processor nor it's flags
-Change
-    The description for AMD Bulldozer CPU was added
-Result
-    Everyone can make the use of this processor and it's flags

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-2012-0748.html