accel/tcg: Move qemu_plugin_vcpu_init__async() to plugins/

accel/tcg: Move qemu_plugin_vcpu_init__async() to plugins/

Calling qemu_plugin_vcpu_init__async() on the vCPU thread
is a detail of plugins, not relevant to TCG vCPU management.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240606124010.2460-4-philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240705084047.857176-30-alex.bennee@linaro.org>