target/ppc: add support for Hypervisor Facility Unavailable Exception

target/ppc: add support for Hypervisor Facility Unavailable Exception

The privileged message send and clear instructions (msgsndp & msgclrp)
are privileged, but will generate a hypervisor facility unavailable
exception if not enabled in the HFSCR and executed in privileged
non-hypervisor state.

Add checks when accessing the DPDES register and when using the
msgsndp and msgclrp isntructions.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20200120104935.24449-3-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>