Clean up MMIO TLB handling.

Clean up MMIO TLB handling.
The IO index is now stored in its own field, instead of being wedged
into the vaddr field.  This eliminates the ROMD and watchpoint host
pointer weirdness.  The IO index space is expanded by 1 bit, and
several additional bits are made available in the TLB vaddr field.

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4704 c046a42c-6fe2-441c-8c8c-71466251a162