virshtest: Prepare for testing against output files

virshtest: Prepare for testing against output files

Modify testCompareOutputLit to take a filename argument and compare it
against and populate the arguments.

For tests which don't use the 'data' from virTestRun, we'll expect to
pass the output filename, thus we also propagate it.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>