ppc64: test rtc parameter

Add a test using get-time-of-day to check "-rtc base=XXXX-XX-XX"
is correct.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>