Re: [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.