test: port postcopy test to ppc64

As userfaultfd syscall is available on powerpc, migration
postcopy can be used.

This patch adds the support needed to test this on powerpc,
instead of using a bootsector to run code to modify memory,
we use a FORTH script in "boot-command" property.

As spapr machine doesn't support "-prom-env" argument
(the nvram is initialized by SLOF and not by QEMU),
"boot-command" is provided to SLOF via a file mapped nvram
(with "-drive file=...,if=pflash")

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
