x86/mem-paging: use guest handle for XENMEM_paging_op_prep

x86/mem-paging: use guest handle for XENMEM_paging_op_prep

While it should have been this way from the beginning, not doing so will
become an actual problem with PVH Dom0. The interface change is binary
compatible, but requires tools side producers to be re-built.

Drop the bogus/unnecessary page alignment restriction on the input
buffer at the same time.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wl@xen.org>