KVM: PPC: Emulate segment fault

Book3S_32 doesn't know about segment faults. It only knows about page faults.
So in order to know that we didn't map a segment, we need to fake segment
faults.

We do this by setting invalid segment registers to an invalid VSID and then
check for that VSID on normal page faults.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>