KVM: PPC: Book3S HV: Implement real mode H_PAGE_INIT handler

Implement a real mode handler for the H_CALL H_PAGE_INIT which can be
used to zero or copy a guest page. The page is defined to be 4k and must
be 4k aligned.

The in-kernel real mode handler halves the time to handle this H_CALL
compared to handling it in userspace for a hash guest.

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