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>