Generic x86 emulator now properly supports 16-bit addressing

Generic x86 emulator now properly supports 16-bit addressing
by narrowing accesses to SI/DI/SP registers. Also supports
real-mode addressing by shifting and adding the appropriate
segment register for certain stack and string operations
where the effective address is not already known.

todo: Stack operations should have address size specified
by B bit in stack segment descriptor, not by default address
size or address-size override. Probably nothing depends on
the proper behaviour though.

Signed-off-by: Keir Fraser <keir@xensource.com>