[Libguestfs] [PATCH] customize: Test that the --root-password parameter works.

This is the second half of the patch series posted yesterday (see: "[PATCH 0/2] builder: Add test that a serial console is set up for guest templates we supply."). This checks that our code to change passwords works on real guests. It creates a real guest using virt-builder with the --root-password option, then boots it up and tries to log in with the root password. Interaction with the guest is afforded by the perl Expect module. Rich.