plugins: Add register write API

plugins: Add register write API

This patch adds a function to the plugins API to allow plugins to write
register contents. It also moves the qemu_plugin_read_register function
so all the register-related functions are grouped together in the file.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Rowan Hart <rowanbhart@gmail.com>
Message-ID: <20250624175351.440780-3-rowanbhart@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20250627112512.1880708-7-alex.bennee@linaro.org>