[Libvir] [PATCH] Tidy up the error messages produced by test driver

This minor patch: * Introduces a VIR_FROM_TEST class of errors and makes errors generated by test come from that class. * Provides a reasonable error message if the user forgets to supply a path. Also stops the driver from trying to open ("/"). Example: $ src/virsh -c test:/// capabilities libvir: Test error : invalid argument in testOpen: supply a path or use test:///default lt-virsh: error: failed to connect to the hypervisor * Arranges that Xen-unified appears last in the list of drivers, because qemu and test have well-defined URI syntaxes so can be checked first. 'make check' passes all tests with this patch. Rich. -- Emerging Technologies, Red Hat http://et.redhat.com/~rjones/ 64 Baker Street, London, W1U 7DF Mobile: +44 7866 314 421 Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SL4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 Directors: Michael Cunningham (USA), Charlie Peters (USA) and David Owens (Ireland)