KVM: PPC: Book3S HV: Shared pages support for secure guests

A secure guest will share some of its pages with hypervisor (Eg. virtio
bounce buffers etc). Support sharing of pages between hypervisor and
ultravisor.

Shared page is reachable via both HV and UV side page tables. Once a
secure page is converted to shared page, the device page that represents
the secure page is unmapped from the HV side page tables.

Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>