ruby: single physical memory in fs mode

ruby: single physical memory in fs mode
Both ruby and the system used to maintain memory copies.  With the changes
carried for programmed io accesses, only one single memory is required for
fs simulations.  This patch sets the copy of memory that used to reside
with the system to null, so that no space is allocated, but address checks
can still be carried out.  All the memory accesses now source and sink values
to the memory maintained by ruby.