x86/HVM: simplify do_hvm_op()

x86/HVM: simplify do_hvm_op()

- boundary checks in HVMOP_modified_memory, HVMOP_set_mem_type, and
  HVMOP_set_mem_access: all of these already check for overflow, so
  there's no need to range check the first _and_ last PFN (checking
  the last one suffices)
- copying back interface structures that were previously copied from
  guest memory can use __copy_to_...(), since copy_from_...() already
  did the address range validation

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>