Can't boot SeaBIOS based VM when using -display gtk, works fine with vnc or sdl
Description of problem:
When using -display gtk, SeaBIOS hangs nondeterministicly. Changing to -display sdl or -display vnc lets it boot.
Steps to reproduce:
1. Run `qemu-system-x86_64 -display gtk` and the VM will not complete BIOS POST.
2. Run `qemu-system-x86_64 -display sdl` and the VM will complete BIOS POST.
Additional information:
This ONLY happens with SeaBIOS. Using a UEFI BIOS to boot the VM does not cause this issue. 

I realise this is a crazy bug. I suspect that the only way it could have slipped through testing is because it *requires* human interaction.

There is no difference with using --accel kvm or not, but I have provided the smallest possible command line to duplicate the issue, which is literally just `qemu-system-x86_64 -display gtk`

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