KVM: PPC: Disable MSR_FEx for Cell hosts

Cell can't handle MSR_FE0 and MSR_FE1 too well. It gets dog slow.
So let's just override the guest whenever we see one of the two and mask them
out. See commit ddf5f75a for reference.

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