boot-serial-test: Add a test for the powernv machine

The new powernv machine ships with a firmware that outputs
some text to the serial console, so we can automatically
test this machine type in the boot-serial tester, too.
And to get some (very limited) test coverage for the new
POWER9 CPU emulation, too, this test is also started with
"-cpu POWER9".

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>