Ah, it's a special serial console boot iso. I was trying the normal NetBSD-<version>-amd64.iso.
So, it seems seabios sercon and bootloader are fighting over the serial line.
seabios enables sercon for no-graphical guests ("-machine graphics=off", "-nographics" enables this too).
So one option is to turn off seabios sercon: "qemu -nographic -machine graphics=on".
The other option is to turn on seabios sercon and use the normal boot.iso (this needs the "-vga none" workaround from comment 3, or the sercon patch).
Ah, it's a special serial console boot iso. I was trying the normal NetBSD- <version> -amd64. iso.
So, it seems seabios sercon and bootloader are fighting over the serial line.
seabios enables sercon for no-graphical guests ("-machine graphics=off", "-nographics" enables this too).
So one option is to turn off seabios sercon: "qemu -nographic -machine graphics=on".
The other option is to turn on seabios sercon and use the normal boot.iso (this needs the "-vga none" workaround from comment 3, or the sercon patch).