tests/migration: Speed up the test on ppc64

tests/migration: Speed up the test on ppc64

The SLOF boot process is always quite slow ... but we can speed it up
a little bit by specifying "-nodefaults" and by using the "nvramrc"
variable instead of "boot-command" (since "nvramrc" is evaluated earlier
in the SLOF boot process than "boot-command").

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>