public/domctl: Fix the struct xen_domctl ABI in 32bit builds

public/domctl: Fix the struct xen_domctl ABI in 32bit builds

The Xen domctl ABI currently relies on the union containing a field with
alignment of 8.

32bit projects which only copy the used subset of functionality end up with an
ABI breakage if they don't have at least one uint64_aligned_t field copied.

Insert explicit padding, and some build assertions to ensure it never changes
moving forwards.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>