qemu patches are discussed & reviewed upstream at the moment.

Plan is to send out qmp events on suspend and wakeup and also add a system_wakeup monitor command which can be used to kick the guest out of s3 state.  libvirt needs to handle those for proper s3 support.

submitted qemu patches:
monitor command: http://patchwork.ozlabs.org/patch/140404/
qmp events: http://patchwork.ozlabs.org/patch/140598/

v2 posted to upstream.

http://www.redhat.com/archives/libvir-list/2012-March/msg00616.html

Per comment 11, move to POST.

The upstream pathches were commited march 23, 11 patches set
a26a1969c37c865b18294c5717544c1dc90beb3c
Add support for event tray moved of removable disks
etc...
Daniel

verify with:
libvirt-0.10.0-0rc0.el6.x86_64
qemu-kvm-0.12.1.2-2.295.el6.x86_64
seabios-0.6.1.2-19.el6.x86_64

step:
  1:prepare a guest with xml
  ......
    <os>
    <type arch='x86_64' machine='rhel6.3.0'>hvm</type>
    <loader>/usr/share/seabios/bios-pm.bin</loader>
    <boot dev='hd'/>
  </os>
  ......
  2: start the guest ,
    in guest run
  #pm-suspent or #echo mem > /sys/power/state
   the guest suspent successful
 #virsh list --all
  Id    Name                           State
----------------------------------------------------
 15    tests3                         pmsuspended

  3:wake up the guest 
   1)use clicking keyboard , the guest wakeup w/o error
   2)use clicking ps/2 mouse ,the guest wakeup w/o error
   3)use monitor command, run in host:
     # virsh qemu-monitor-command tests3 '{"execute":"system_wakeup"}'
   {"return":{},"id":"libvirt-43"}
   the guest can wakeup w/o error.
  4: use virsh command to into s3
    install vdagent in guest os.
   add qemu-ga support in guest xml:
   <channel type='unix'>
      <source mode='bind' path='/var/lib/libvirt/qemu/tests3.agent'/>
      <target type='virtio' name='org.qemu.guest_agent.0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>
  in guest run #qemu-ga -d
  in host run:
  #virsh dompmsuspend $guest_name --target mem
  Domain tests3 successfully suspended
  #virsh domstate $guest_name
  pmsuspended

   wakeup guest:
  #virsh dompmwakeup $guest_name
  Domain tests3 successfully woken up
  

verification passed.

Now, in pkg qemu-guest-agent-win32-0.12.1.2-2.346.el6.x86_64.rpm  we can do the testing in windows guest based the windows guest agent supported commands list
supported commands:
"guest-info"
"guest-ping"
"guest-sync-delimited"
"guest-sync"
"guest-shutdown" (include shutdown,reboot,halt)
"guest-suspend-disk"
"guest-suspend-ram"
Since there was a bug 888716 over there ,so the command "guest-susend-disk "not work well so far , besides,qemu agen didn't support  "guest-suspend-mem" yet,so we can't test it untile the bug fixed and qemu agent support it

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-2013-0276.html