lib: Don't use virReportSystemError() if virCommandRun() fails

lib: Don't use virReportSystemError() if virCommandRun() fails

Firstly, virCommandRun() does report an error on failure (which
in most cases is more accurate than what we overwrite it with).
Secondly, usually errno is not set (or gets overwritten in the
cleanup code) which makes virReportSystemError() report useless
error messages. Drop all virReportSystemError() calls in cases
like this (I've found three occurrences).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>