xen/mm: Clean up share_xen_page_with_guest() API

xen/mm: Clean up share_xen_page_with_guest() API

The share_xen_page_with_guest() functions are used by common code, and are
implemented the same by each arch.  Move the declarations into the common mm.h
rather than duplicating them in each arch/mm.h

Turn an int readonly into a boolean enum, to retain ro/rw context at the
callsites, but use shorter labels which avoids a large number of split lines.

Implement share_xen_page_with_privileged_guests() as a static inline wrapper
around share_xen_page_with_guest() to avoid having a call into a separate
translation unit whose only purpose is to shuffle function arguments.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>