hw/core/generic-loader: Allow PC to be set on command line

The documentation for the generic loader claims that you can
set the PC for a CPU with an option of the form
  -device loader,cpu-num=0,addr=0x10000004

However if you try this QEMU complains:
  cpu_num must be specified when setting a program counter

This is because we were testing against 0 rather than CPU_NONE.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180205150426.20542-1-peter.maydell@linaro.org