rescue: Add an expect-driven test for the virt-rescue command.

This command was not tested at all.  As a result we didn't notice that
it was broken for a long time (RHBZ#853159).

This adds a test that drives the command through a pty.  It uses the
perl 'Expect' module, although this is not required.

README

@@ -113,6 +113,9 @@ For basic functionality and the C tools:
 - netpbm, icoutils (optional)
   These programs are used to render icons from guests.
 
+- Perl Expect module (optional)
+  This is used to test virt-rescue.
+
 To build FUSE support in the core library, and guestmount:
 
 - FUSE libraries and kernel module (optional)