virt: vbox: struct vmmdev_hgcm_pagelist: Replace 1-element array with flexible array

Replace the deprecated[1] use of a 1-element array in
struct vmmdev_hgcm_pagelist with a modern flexible array. As this is
UAPI, we cannot trivially change the size of the struct, so use a union
to retain the old first element's size, but switch "pages" to a flexible
array.

No binary differences are present after this conversion.

Link: https://github.com/KSPP/linux/issues/79

 [1]
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20240710231555.work.406-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>