This feature is now implemented upstream by v0.9.9-140-gb66d1be and v0.9.9-139-g5a137f3:

commit b66d1bef14739ab56ed9f430b4c4e5b89ed5ed12
Author: Paolo Bonzini <pbonzini>
Date:   Fri Jan 27 14:49:52 2012 +0100

    qemu: parse and create -cpu ...,-kvmclock
    
    QEMU supports a bunch of CPUID features that are tied to the kvm CPUID
    nodes rather than the processor's.  They are "kvmclock",
    "kvm_nopiodelay", "kvm_mmu", "kvm_asyncpf".  These are not known to
    libvirt and their CPUID leaf might move if (for example) the Hyper-V
    extensions are enabled. Hence their handling would anyway require some
    special-casing.
    
    However, among these the most useful is kvmclock; an additional
    "property" of this feature is that a <timer> element is a better model
    than a CPUID feature.  Although, creating part of the -cpu command-line
    from something other than the <cpu> XML element introduces some
    ugliness.

commit 5a137f3620af4f6fd4a4484c6d79af0748aec85e
Author: Paolo Bonzini <pbonzini>
Date:   Fri Jan 27 14:49:51 2012 +0100

    conf: add kvmclock timer
    
    Add kvmclock timer to documentation, schema and parsers.  Keep the
    platform timer first since it is kind of special, and alphabetize
    the others when possible (i.e. when it does not change the ABI).

Verified this bug with libvirt-0.9.10-1.el6
Steps:
1. For a shutdown guest,
# virsh edit guest
...
  <clock offset='utc'>
    <timer name='kvmclock' present='yes'/>
  </clock>
...

2. # virsh start guest
   # ps -ef |grep kvm
   qemu     12826     1 43 04:56 ?        00:00:20 /usr/libexec/qemu-kvm -S -M rhel6.2.0 -cpu qemu64,+kvmclock -enable-kvm -m 1024 -smp 1,sockets=1,cores=1,threads=1 -name rhel62......

You can find '-cpu qemu64,+kvmclock' from qemu cmd. Also login the guest and check
# dmesg|grep kvm
kvm-clock: Using msrs 4b564d01 and 4b564d00
kvm-clock: cpu 0, msr 0:1c1df41, boot clock
kvm-clock: cpu 0, msr 0:2215f41, primary cpu clock
Switching to clocksource kvm-clock


3. # virsh destroy guest
   # virsh edit guest
...
  <clock offset='utc'>
    <timer name='kvmclock' present='no'/>
  </clock>
...

4. # virsh start guest
   # ps -ef |grep kvm
qemu     13115     1 13 04:58 ?        00:00:00 /usr/libexec/qemu-kvm -S -M rhel6.2.0 -cpu qemu64,-kvmclock -enable-kvm -m 1024 -smp 1,sockets=1,cores=1,threads=1 -name rhel62
......

You can find '-cpu qemu64,-kvmclock' from qemu cmd. Also login the guest and check
# dmesg|grep kvm
nothing return.

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