Verified the issue on 2.6.32-131.21.1.el6.x86_64 with the same steps in:
https://bugzilla.redhat.com/show_bug.cgi?id=740465#c0
The issue does not exist anymore, guest works well after several times UDP netperf stress test. So this bug is fixed.
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-2011-1465.html