plugins: extract generate ptr for qemu_plugin_u64

plugins: extract generate ptr for qemu_plugin_u64

Plugin operations can access a scoreboard. This function factorizes code
generation for accessing entry associated to a given vcpu.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240502211522.346467-3-pierrick.bouvier@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240514174253.694591-5-alex.bennee@linaro.org>