KVM: PPC: Book3S HV: Allow passthrough of an emulated device to an L3 guest

Previously when a device was being emulated by an L1 guest for an L2
guest, that device couldn't then be passed through to an L3 guest. This
was because the L1 guest had no method for accessing L3 memory.

The hcall H_COPY_TOFROM_GUEST provides this access. Thus this setup for
passthrough can now be allowed.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>