xen/memory: Improve compat XENMEM_acquire_resource handling

xen/memory: Improve compat XENMEM_acquire_resource handling

The frame_list is an input, or an output, depending on whether the calling
domain is translated or not.  The array does not need marshalling in both
directions.

Furthermore, the copy-in loop was very inefficient, copying 4 bytes at at
time.  Rewrite it to copy in all nr_frames at once, and then expand
compat_pfn_t to xen_pfn_t in place.

Re-position the copy-in loop to simplify continuation support in a future
patch, and reduce the scope of certain variables.

No change in guest observed behaviour.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>