system/vl: Filter machine list available for a particular target binary

system/vl: Filter machine list available for a particular target binary

Binaries can register a QOM type to filter their machines
by filling their TargetInfo::machine_typename field.

This can be used by example by main() -> machine_help_func()
to filter the machines list.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>