Description of problem:
While starting a vm on rhel 6.3 Beta (which itself is a vm), libvirtd crashes

Version-Release number of selected component (if applicable):
libvirt-0.9.10-11.el6.x86_64

How reproducible:
every time

Steps to Reproduce:
1. create a rhel 6.3 vm
2. using this vm as a host attempt to start another vm
3.
  
Actual results:
libvird crashes

Expected results:
nested vm should start

Additional info:

this symbolic link was created on the virtual host 
/usr/bin/qemu-system-x86_64 -> /usr/libexec/qemu-kvm

output from gdb attached to libvirtd
Program received signal SIGSEGV, Segmentation fault.
0x0000000000476292 in qemuBuildCpuArgStr (conn=0x7f0118000bd0, driver=0x7f0128006430, def=0x7f0128133590, monitor_chr=0x7f012804e5d0, monitor_json=true, qemuCaps=0x7f0128133a30, migrateFrom=0x0, migrateFd=-1,
    snapshot=0x0, vmop=VIR_NETDEV_VPORT_PROFILE_OP_CREATE) at qemu/qemu_command.c:3824
3824        cpuDataFree(host->arch, data);



As discussed with danpb this is fixed upstream

commit 2ccc4a607f6e122aff2e3b9d133d6e6b4b661a1e
Author: Jiri Denemark <jdenemar@redhat.com>
Date:   Wed Feb 15 12:18:25 2012 +0100

    qemu: Fix segfault when host CPU is empty
    
    In case libvirtd cannot detect host CPU model (which may happen if it
    runs inside a virtual machine), the daemon is likely to segfault when
    starting a new qemu domain. It segfaults when domain XML asks for host
    (either model or passthrough) CPU or does not ask for any specific CPU
    model at all.