tcg plugins: expose an API version concept

tcg plugins: expose an API version concept

This is a very simple versioning API which allows the plugin
infrastructure to check the API a plugin was built against. We also
expose a min/cur API version to the plugin via the info block in case
it wants to avoid using old deprecated APIs in the future.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Robert Foley <robert.foley@linaro.org>