KVM: powerpc: fix init/exit annotation

kvmppc_e500_exit() is a module_exit function, so it should be tagged
with __exit, not __init. The incorrect annotation was added by commit
2986b8c7.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@kernel.org
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>