pseries: Add support for new KVM hash table control call

This adds support for then new "reset htab" ioctl which allows qemu
to properly cleanup the MMU hash table when the guest is reset. With
the corresponding kernel support, reset of a guest now works properly.

This also paves the way for indicating a different size hash table
to the kernel and for the kernel to be able to impose limits on
the requested size.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
