KVM: PPC: Fix mfspr/mtspr MMUCFG emulation

On mfspr/mtspr emulation path Book3E's MMUCFG SPR with value 1015 clashes
with G4's MSSSR0 SPR. Move MSSSR0 emulation from generic part to Books3S.
MSSSR0 also clashes with Book3S's DABRX SPR. DABRX was not explicitly
handled so Book3S execution flow will behave as before.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>