cpu: introduce CPUClass::virtio_is_big_endian()

cpu: introduce CPUClass::virtio_is_big_endian()

If we want to support targets that can change endianness (modern PPC and
ARM for the moment), we need to add a per-CPU class method to be called
from the virtio code. The virtio_ prefix in the name is a hint for people
to avoid misusage (aka. anywhere but from the virtio code).

The default behaviour is to return the compile-time default target
endianness.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>