libvirt.spec has:

  # We want to install the default network for initial RPM installs
  # or on the first upgrade from a non-network aware libvirt only.
  # We check this by looking to see if the daemon is already installed
  /sbin/chkconfig --list libvirtd 1>/dev/null 2>&1
  if [ $? != 0 ]
  then
    UUID=`/usr/bin/uuidgen`
    sed -e "s,</name>,</name>\n  <uuid>$UUID</uuid>," \
         < /usr/share/libvirt/networks/default.xml \
         > /etc/libvirt/qemu/networks/default.xml
    ln -s ../default.xml /etc/libvirt/qemu/networks/autostart/default.xml
  fi
  /sbin/chkconfig --add libvirtd


This worked on F-10 and before because chkconfig would return non-zero if this is the first time libvirt is installed because 'chkconfig --add libvirtd' has not been run yet

i.e. before --add, F-10 chkconfig gives:

  $> chkconfig --list libvirtd
  service libvirtd supports chkconfig, but is not referenced in any runlevel (run 'chkconfig --add libvirtd')
  $> echo $?
  1

however, on F-11 chkconfig gives:

  $> chkconfig --list libvirtd
  libvirtd       	0:off	1:off	2:off	3:off	4:off	5:off	6:off
  $> echo $?
  0

Upshot is - if you install current rawhide, there is no libvirt default network and no external network connectivity for guests by default.