tests/plugins: use qemu_plugin_outs for inline stats

tests/plugins: use qemu_plugin_outs for inline stats

Using bare printf's in plugins is perfectly acceptable but they do
rather mess up the output of "make check-tcg". Convert the printfs to
use g_string and then output with the plugin output helper which will
already be captured to .pout files by the test harness.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240718094523.1198645-7-alex.bennee@linaro.org>