Specifying ALSA subdevice

[Olli Helin]

This was asked already a couple of years ago but no solution was found.

I would like to use an ALSA loopback device for my virtual machine. With the 
now deprecated soundhw/env parameter syntax everything worked. I'm specifying 
"-device intel-hda -device hda-duplex " which are fine I guess, but the 
-audiodev parameter is causing trouble:

        -audiodev 
alsa,id=snd0,out.dev='hw:Loopback,0,4',in.dev='hw:Loopback,1,5'

results in the following error: Invalid parameter '0'

This is logical as comma is the parameter separator. But apparently it cannot 
be escaped in any way?

I tried using the JSON form:

        -audiodev \"{
            'driver':'alsa',
            'id':'snd0',
            'out':{'dev':'hw:Loopback,0,4'},
            'in':{'dev':'hw:Loopback,1,5'}
        }\"

but this resulted in the following error: qemu-system-x86_64: -audiodev "{: 
Parameter 'id' is missing

This I do not understand, as there clearly is the id property for the audiodev.

Is there a bug in the JSON parsing, is using the HW subdevices not possible, or 
am I missing something? Thank you!