Description of problem:

There should be a better error message when tun is not loaded.


Version-Release number of selected component (if applicable):

0.500.2

Steps to Reproduce:
1. Don't have tun loaded into the kernel
2. try running virt-install
3.
  
Actual results:

Starting install...
ERROR    Failed to add tap interface to bridge 'br0': No such file or directory
Domain installation may not have been
 successful.  If it was, you can restart your domain
 by running 'virsh start tomcat'; otherwise, please
 restart your installation.
ERROR    Failed to add tap interface to bridge 'br0': No such file or directory
Traceback (most recent call last):
  File "/usr/bin/virt-install", line 972, in <module>
    main()
  File "/usr/bin/virt-install", line 834, in main
    start_time, guest.start_install)
  File "/usr/bin/virt-install", line 896, in do_install
    dom = install_func(conscb, progresscb, wait=(not wait))
  File "/usr/lib/python2.6/site-packages/virtinst/Guest.py", line 795, in start_install
    return self._do_install(consolecb, meter, removeOld, wait)
  File "/usr/lib/python2.6/site-packages/virtinst/Guest.py", line 896, in _do_install
    self.domain = self.conn.createLinux(install_xml, 0)
  File "/usr/lib/python2.6/site-packages/libvirt.py", line 1147, in createLinux
    if ret is None:raise libvirtError('virDomainCreateLinux() failed', conn=self)
libvirtError: Failed to add tap interface to bridge 'br0': No such file or directory

Expected results:

Tell the user to load the tun kernel module.