plugins: add qemu_plugin_num_vcpus function

plugins: add qemu_plugin_num_vcpus function

We now keep track of how many vcpus were started. This way, a plugin can
easily query number of any vcpus at any point of execution, which
unifies user and system mode workflows.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240213094009.150349-3-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240227144335.1196131-17-alex.bennee@linaro.org>