plugins: add an API to read registers

plugins: add an API to read registers

We can only request a list of registers once the vCPU has been
initialised so the user needs to use either call the get function on
vCPU initialisation or during the translation phase.

We don't expose the reg number to the plugin instead hiding it behind
an opaque handle. For now this is just the gdb_regnum encapsulated in
an anonymous GPOINTER but in future as we add more state for plugins
to track we can expand it.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1706
Based-on:  <20231025093128.33116-18-akihiko.odaki@daynix.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240227144335.1196131-24-alex.bennee@linaro.org>