target-ppc: ppc64 target's virtio can be either endian

This is the enablement patch to support endianness change with PPC64
guests.

We implement the ppc_has_big_endian_interrupts() for POWER7 and up thanks
to the ILE bit of the LPCR spr (interrupt vectors endianness).

We also define TARGET_BIENDIAN to select appropriate virtio accessors.

Suggested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
[ original hook superseded by CPU method,
  Greg Kurz <gkurz@linux.vnet.ibm.com> ]
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Alexander Graf <agraf@suse.de>