semantic: 0.583
mistranslation: 0.566
x86: 0.496
peripherals: 0.457
device: 0.455
graphic: 0.370
ppc: 0.317
socket: 0.196
architecture: 0.167
user-level: 0.155
risc-v: 0.139
kernel: 0.137
i386: 0.137
VMM: 0.133
performance: 0.115
hypervisor: 0.113
network: 0.109
vnc: 0.091
arm: 0.090
virtual: 0.084
permissions: 0.070
boot: 0.066
KVM: 0.062
register: 0.059
assembly: 0.056
debug: 0.041
PID: 0.038
TCG: 0.037
files: 0.008

serial/parallel character devices created for the none-machine

The none-machine can not be started unless using "-serial null":

qemu-system-x86_64 -machine none -nographic -monitor stdio
QEMU 3.1.1 monitor - type 'help' for more information
(qemu) qemu-system-x86_64: cannot use stdio by multiple character devices
qemu-system-x86_64: could not connect serial device to character backend 'stdio'
$

$ qemu-system-mips -machine none -nographic -serial null -monitor stdio
QEMU 4.1.50 monitor - type 'help' for more information
(qemu) info chardev
parallel0: filename=null
compat_monitor0: filename=stdio
serial0: filename=null
(qemu)

You can start 'none' without "-serial null". Examples:

qemu-system-x86_64 -machine none
qemu-system-x86_64 -machine none -monitor stdio
qemu-system-x86_64 -machine none -nographic
qemu-system-x86_64 -machine none -monitor stdio -display none

Your command line "qemu-system-x86_64 -machine none -nographic -monitor stdio" fails because "-nographic" says "please create a serial port using stdio" but "-monitor stdio" tries to use stdio for something else. You get the same message for any machine (eg "pc"), not just "none". If what you wanted was "just don't create the graphical display" that's "-display none" -- "-nographic" is a collection of things including both 'no display' and also 'default to creating a serial device to stdio' and 'default to creating a monitor muxed with that serial'.