xen: CFI hardening for custom_param()

xen: CFI hardening for custom_param()

Control Flow Integrity schemes use toolchain and optionally hardware support
to help protect against call/jump/return oriented programming attacks.

Use cf_check to annotate function pointer targets for the toolchain.

The "watchdog_timeout" and "cpu_type" handlers were missing __init.

The "numa", "acpi", "irq_vector_map" and "flask" handlers can skip forward
declarations by altering the custom_param() position.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>