plugins: add missing export for qemu_plugin_num_vcpus

plugins: add missing export for qemu_plugin_num_vcpus

Fixes: 4a448b148ca ("plugins: add qemu_plugin_num_vcpus function")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20241112212622.3590693-2-pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241121165806.476008-36-alex.bennee@linaro.org>
(cherry picked from commit cfa3a6c54511374e9ccee26d9c38ac1698fc7af2)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>