QEMU ppce500 board uses the legacy -serial option.

Other PPC boards don't give any way to explicitly wire in a -chardev
except pseries which uses -device spapr-vty with -chardev.

Add test case for -serial option for ppce500
Signed-off-by: NOlivia Yin <Hong-Hua.Yin@freescale.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>