PPC: Add PV hypercall transport through fw_cfg

On KVM for PPC we need to tell the guest which instructions to use when
doing a hypercall. The clean way to do this is to go through an ioctl
from userspace and passing it on to the guest using the device tree.

So let's do the qemu part here: read out the hypercall and pass it on
to the guest's fw_cfg so openBIOS can read it out and expose it again.

Signed-off-by: Alexander Graf <agraf@suse.de>
