Enhance guest memory accessor macros so that source operands can be

Enhance guest memory accessor macros so that source operands can be
pointers to const or arrays.

Only build-tested on ia64, and untested for powerpc (which, however,
is almost identical to ia64, except for an apparent bug in the original
version of __copy_field_{from,to}_guest in that the field offset was
multiplied by the field size).

Signed-off-by: Jan Beulich <jbeulich@novell.com>