tests/unit/test-qga: do not qualify executable paths

tests/unit/test-qga: do not qualify executable paths

guest-exec invocation does not need the full path of the executable to
execute. Using only the command names ensures correct execution of the
test on systems not adhering to the FHS.

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>