stub: Prefer graphical console over virtio detection heuristic

stub: Prefer graphical console over virtio detection heuristic

With vmspawn --console=gui and similar one has the case where the VirtIO
PCI device is present for something else than the console and the logic
in sd-stub added console=hvc0 even though it didn't exist. This caused
tty0 to be empty during boot.
Always prefer the graphical console before a potential virtio console.
The virtio console is still preferred over a serial console and we can
get the same problem there, e.g., qemu-guest-agent is used causing the
PCI device but without a virtio console and instead a serial console is
used. This is not solved here.