device_memory_interface::space() assert against NULL and
return a reference, and pushed references throughout all
address space usage in the system. Added a has_space()
method to check for those rare case when it is ambiguous.
[Aaron Giles]
Also reinstated the generic space and added fatal error
handlers if anyone tries to actually read/write from it.
device_execute/memory_interfaces respectively in order
to catch unnecessary usage of the corresponding device_t
methods. Removed all existing redundant usage.
[Aaron Giles]
