test driver: Add authentication to test driver.

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>