xen/include: public: Document the padding in struct xen_hvm_param

xen/include: public: Document the padding in struct xen_hvm_param

There is an implicit padding of 2 bytes in struct xen_hvm_param between
the field domid and index. Make it explicit by introduce a padding
field. This can also serve as documentation.

Note that I don't think we can mandate it to be zero because a guest may
not have initialized the padding.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wl@xen.org>