plugins: add time control API

plugins: add time control API

Expose the ability to control time through the plugin API. Only one
plugin can control time so it has to request control when loaded.
There are probably more corner cases to catch here.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
[AJB: tweaked user-mode handling, merged QEMU_PLUGIN_API fix]
Message-Id: <20240530220610.1245424-6-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240620152220.2192768-9-alex.bennee@linaro.org>