plugins: remove previous n_vcpus functions from API

plugins: remove previous n_vcpus functions from API

This information is already accessible using qemu_info_t during plugin
install.

We will introduce another function (qemu_plugin_num_vcpus) which
represent how many cpus were enabled, by tracking new cpu indexes.

It's a breaking change, so we bump API version.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240213094009.150349-2-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240227144335.1196131-16-alex.bennee@linaro.org>