The debian netinst is able to work over serial but if a graphic card is detected, the bootloader won't be sent over serial, which make appending options to display over serial a bit tricky. And of course, when booting a qemu with -nographic, it doesn't remove the graphic card.
The trick here is to use the monitor to send keys to the bootloader. You just have to send the keys to tell the kernel and the install to boot and start using ttyS0. Which means that you need to write the following in the monitor :
sendkey i sendkey n sendkey s sendkey t sendkey a sendkey l sendkey l sendkey spc sendkey c sendkey o sendkey n sendkey s sendkey o sendkey l sendkey e sendkey equal sendkey t sendkey t sendkey y sendkey shift-s sendkey 0 sendkey ret
That will start the image install with argument console=ttyS0.