plugins/api: use qemu_target_page_mask() to get value

plugins/api: use qemu_target_page_mask() to get value

Requiring TARGET_PAGE_MASK to be defined gets in the way of building
this unit once. qemu_target_page_mask() will tell us what it is.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250304222439.2035603-21-alex.bennee@linaro.org>