KVM: PPC: Don't export Book3S symbols on BookE

Book3S knows how to convert floats to doubles and vice versa. BookE doesn't.
So let's make sure we don't export them on BookE.

This fixes a link error on BookE with CONFIG_KVM=y.

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