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: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NAlistair Francis <alistair.francis@xilinx.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180205150426.20542-1-peter.maydell@linaro.org

hw/core/generic-loader.c

@@ -105,7 +105,7 @@ static void generic_loader_realize(DeviceState *dev, Error **errp)
             error_setg(errp, "data can not be specified when setting a "
                        "program counter");
             return;
-        } else if (!s->cpu_num) {
+        } else if (s->cpu_num == CPU_NONE) {
             error_setg(errp, "cpu_num must be specified when setting a "
                        "program counter");
             return;