I have a new Sun fire X4140 server with two amd opteron 2435 CPUs running debian jessie, libvirt 1.2.9-3, virtinst 1.0.1-3, qemu/kvm 2.1. When attempting to create virtual machines (with --debug), I receive this:

[Thu, 20 Nov 2014 13:56:36 virt-install 1842] DEBUG (cli:234)   File "/usr/share/virt-manager/virt-install", line 876, in <module>
    sys.exit(main())
  File "/usr/share/virt-manager/virt-install", line 870, in main
    start_install(guest, continue_inst, options)
  File "/usr/share/virt-manager/virt-install", line 588, in start_install
    fail(e, do_exit=False)
  File "/usr/share/virt-manager/virtinst/cli.py", line 234, in fail
    logging.debug("".join(traceback.format_stack()))

[Thu, 20 Nov 2014 13:56:36 virt-install 1842] ERROR (cli:235) internal error: Cannot find suitable CPU model for given data
[Thu, 20 Nov 2014 13:56:36 virt-install 1842] DEBUG (cli:237)
Traceback (most recent call last):
  File "/usr/share/virt-manager/virt-install", line 560, in start_install
    dom = guest.start_install(meter=meter, noboot=options.noreboot)
  File "/usr/share/virt-manager/virtinst/guest.py", line 403, in start_install
    noboot)
  File "/usr/share/virt-manager/virtinst/guest.py", line 467, in _create_guest
    dom = self.conn.createLinux(start_xml or final_xml, 0)
  File "/usr/lib/python2.7/dist-packages/libvirt.py", line 3440, in createLinux
    if ret is None:raise libvirtError('virDomainCreateLinux() failed', conn=self)
libvirtError: internal error: Cannot find suitable CPU model for given data
[Thu, 20 Nov 2014 13:56:36 virt-install 1842] DEBUG (cli:248) Domain installation does not appear to have been successful.