x86: make PioBus return BadAddress errors

Stop setting the use_default_range flag in PioBus in order to
have random bad addresses result in a BadAddress response and
not a gem5 fatal error.  This is necessary in Ruby as Ruby is
connected directly to PioBus, so misspeculated addresses will
be sent there directly.  For the classic memory system, this
change has no effect, as bad addresses are caught by the
memory bus before being sent to the PioBus.

This work was done while Binh was an intern at AMD Research.

src/dev/x86/Pc.py

@@ -79,4 +79,3 @@ class Pc(Platform):
         self.fake_com_4.pio = bus.master
         self.fake_floppy.pio = bus.master
         self.pciconfig.pio = bus.default
-        bus.use_default_range = True