hw/arm/virt: set stdout-path instead of linux,stdout-path

ePAPR 1.1 defines the stdout-path property, making the os-specific
linux,stdout-path property redundant. Change the DT setup for ARM virt
to use the generic property - supported by Linux since 3.15.

The old QEMU behaviour was not present in any released version of
QEMU, and was only added to QEMU after the kernel changed, so
this should not break any existing setups.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
[PMM: add note to commit about the old behaviour never hving been
in a released version of QEMU]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>