hw/pc.c: Fix converting of ioport_register* to MemoryRegion

hw/pc.c: Fix converting of ioport_register* to MemoryRegion

The commit 258711 introduced MemoryRegion to replace ioport_region*
for ioport 80h and F0h.
A MemoryRegion needs to have both read and write callback otherwise a segfault
will occur when an access is made.

The previous behaviour of this both ioport is to return 0xffffffffffffffff.
So keep this behaviour.

Reported-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Tested-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>