Using KVM, I've not been able to use virt-manager to create a new virtual
machine. It fails setting up the network bridge (I selected the default network
setup, as I'm on a wireless network handled by NetworkManager).

I get this traceback after the disk image has been created:
Unable to complete install '<class 'libvirt.libvirtError'>
virDomainCreateLinux() failed Failed to add tap interface 'vnet-1078634840' to
bridge 'virbr0' : No such device
Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/create.py", line 616, in do_install
    dom = guest.start_install(False, meter = meter)
  File "/usr/lib/python2.5/site-packages/virtinst/Guest.py", line 724, in
start_install
    return self._do_install(consolecb, meter)
  File "/usr/lib/python2.5/site-packages/virtinst/Guest.py", line 741, in
_do_install
    self.domain = self.conn.createLinux(install_xml, 0)
  File "/usr/lib/python2.5/site-packages/libvirt.py", line 573, in createLinux
    if ret is None:raise libvirtError('virDomainCreateLinux() failed', conn=self)
libvirtError: virDomainCreateLinux() failed Failed to add tap interface
'vnet-1078634840' to bridge 'virbr0' : No such device
'

virt-manager-0.5.2-2.fc8
python-virtinst-0.300.1-3.fc8
libvirt-python-0.3.3-2.fc8