semantic: 0.197
graphic: 0.176
other: 0.168
device: 0.165
socket: 0.056
permissions: 0.038
vnc: 0.033
performance: 0.031
network: 0.030
PID: 0.026
boot: 0.023
KVM: 0.022
debug: 0.018
files: 0.018
boot: 0.134
files: 0.129
device: 0.097
debug: 0.091
KVM: 0.076
semantic: 0.072
other: 0.072
graphic: 0.060
PID: 0.060
network: 0.059
socket: 0.051
permissions: 0.038
performance: 0.035
vnc: 0.027

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'.