Reproduced on rhel6.0 guest with kernel:2.6.32-71.el6.x86_64.
Steps:
1) boot guest with 512M mem and virtio net, ping remote, network works fine.
2) run netserver inside guest.
3) on host, launch 2000 netperf clients in background to stress netserver.
#! /bin/sh
ip=$guest_ip
i=0
while [ $i -lt 2000 ]
do
netperf -H $ip -l 300 &
i=`expr $i + 1`
echo launch Client-No.$i 
done
4) ping guest
Actual Result: network lost, fail to ping remote host.
CLI:
/usr/libexec/qemu-kvm -M rhel6.0.0 -enable-kvm -m 512 -smp 2 -name rhel6.0 -uuid `uuidgen` -rtc base=localtime,clock=vm,driftfix=slew -no-kvm-pit-reinjection -boot c -drive file=/root/RHEL-Server-6.0-64.qcow2,if=none,id=drive-virtio-0-0,media=disk,format=qcow2,cache=none -device virtio-blk-pci,drive=drive-virtio-0-0,id=virt0-0-0 -netdev tap,id=hostnet1 -device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:40:01:31:e3 -usb -device usb-tablet,id=input1 -vnc :0 -monitor stdio -balloon none

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Verified on guest kernel-2.6.32-71.23.1.el6.x86_64.rpm with same steps&CLI above, after stressing netserver, network in guest still works fine and can ping remote host.

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHSA-2011-0421.html

    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:
Intensive usage of resources on a guest lead to a failure of networking on that guest: packets could no longer be received. The failure occurred when a DMA (Direct Memory Access) ring was consumed before NAPI (New API; an interface for networking devices which makes use of interrupt mitigation techniques) was enabled which resulted in a failure to receive the next interrupt request. The regular interrupt handler was not affected in this situation (because it can process packets in-place), however, the OOM (Out Of Memory) handler did not detect the aforementioned situation and caused networking to fail. With this update, NAPI is subsequently scheduled for each napi_enable operation; thus, networking no longer fails under the aforementioned circumstances.