x86/boot: Fix latent memory corruption with early_boot_opts_t

x86/boot: Fix latent memory corruption with early_boot_opts_t

c/s ebb26b509f "xen/x86: make VGA support selectable" added an #ifdef
CONFIG_VIDEO into the middle the backing space for early_boot_opts_t,
but didn't adjust the structure definition in cmdline.c

This only functions correctly because the affected fields are at the end
of the structure, and cmdline.c doesn't write to them in this case.

To retain the slimming effect of compiling out CONFIG_VIDEO, adjust
cmdline.c with enough #ifdef-ary to make C's idea of the structure match
the declaration in asm.  This requires adding __maybe_unused annotations
to two helper functions.

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