test driver: Add authentication to test driver.
    
    There is no easy way to test authentication against libvirt.  This
    commit modifies the test driver to allow simple username/password
    authentication.
    
    You modify the test XML by adding:
    
     <node>
       ...
       <auth>
         <user password="123456">rich</user>
         <user>jane</user>
       </auth>
     </node>
    
    If there are any /node/auth/user elements, then authentication is
    required by the test driver (if none are present, then the test driver
    will work as before and not require authentication).
    
    In the example above, two phony users are added:
    
     rich  password: 123456
     jane  no password required
    
    The test driver will demand a username.  If the password attribute is
    present (or if the username entered is wrong), then the password is
    also asked for and checked:
    
     $ virsh -c test://$(pwd)/testnode.xml list
     Enter username for localhost: rich
     Enter rich's password for localhost: ***
      Id    Name                           State
     ----------------------------------------------------
      1     fv0                            running
      2     fc4                            running
    
    Signed-off-by: Richard W.M. Jones <rjones@redhat.com>