xen: use struct_size() helper in kzalloc()

struct privcmd_buf_vma_private has a zero-sized array at the end
(pages), use the new struct_size() helper to determine the proper
allocation size and avoid potential type mistakes.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>