virshtest: use virCommand instead of custom impl

virshtest: use virCommand instead of custom impl

Our virCommand helper API already has the ability to capture
program output, there's no need to open-code it.

Apart from simplifying the code, the test is marginally faster
due to recent improvements in virCommandMassClose.

Until now, both stderr and stdout were stored in the same buffer.
This change stores stderr separately and expects it to be empty
for all the tests we currently run.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>