powerpc 7450 MMU initialization broken

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/812398

The 7540 family of PPCs' MMU can update TLBs using hardware search (like a
604 or 7400) but also using a software algorithm. The mechanism used is
defined by HID0[STEN].

By default (CPU reset) HID0 is set to 0x80000000 (BTW; another small bug,
qemu doesn't set the hardwired MSB), hence
the software-table lookup feature is *disabled*. However, the default (and
immutable) 'mmu_model' for this CPU family is POWERC_MMU_SOFT_74XX which
choses the soft TLB replacement scheme.

To fix this:

1) the initial mmu_model for the 7450 family (includes 7441, 7445, 7451,
7455, 7457, 7447, 7448) should be: POWERPC_MMU_32B
2) when HID0[STEN] is written then the mmu_model should be changed
accordingly (I'm not familiar enough with the qemu internal state to judge
if any cached state would have to be updated).