libvirt-0.8.8-7.fc15.x86_64
qemu-kvm-0.14.0-7.fc15.x86_64

My host is Nehalem class.
Here's a snippet from virsh capabilities

    <cpu>
      <arch>x86_64</arch>
      <model>Nehalem</model>
      <vendor>Intel</vendor>
      <topology sockets='1' cores='2' threads='2'/>
      <feature name='rdtscp'/>
      <feature name='xtpr'/>
      <feature name='tm2'/>
      <feature name='est'/>
      <feature name='vmx'/>
      <feature name='ds_cpl'/>
      <feature name='monitor'/>
      <feature name='pbe'/>
      <feature name='tm'/>
      <feature name='ht'/>
      <feature name='ss'/>
      <feature name='acpi'/>
      <feature name='ds'/>
      <feature name='vme'/>
    </cpu>

I used virt-manager's copy host config option to autodetect the right cpu type (Nehalem)

Looking at the domain XML I see

  <cpu match='exact'>
    <model>Nehalem</model>
    <vendor>Intel</vendor>
    <feature policy='require' name='tm2'/>
    <feature policy='require' name='est'/>
    <feature policy='require' name='monitor'/>
    <feature policy='require' name='ss'/>
    <feature policy='require' name='vme'/>
    <feature policy='require' name='rdtscp'/>
    <feature policy='require' name='ht'/>
    <feature policy='require' name='ds'/>
    <feature policy='require' name='pbe'/>
    <feature policy='require' name='tm'/>
    <feature policy='require' name='vmx'/>
    <feature policy='require' name='ds_cpl'/>
    <feature policy='require' name='xtpr'/>
    <feature policy='require' name='acpi'/>
  </cpu>

But when I start the VM here's what I see in ps


/usr/bin/qemu-kvm -S -M pc-0.14 -cpu core2duo,+lahf_lm,+rdtscp,+popcnt,+sse4.2,+sse4.1,+xtpr,+cx16,+tm2,+est,+vmx,+ds_cpl,+pbe,+tm,+ht,+ss,+acpi,+ds