run-ovmf refuses --second-head with --s3, whose guest has no second output

[DevomB]

What was wrong

tools/image/run-ovmf.sh --gpu --s3 now shows the guest through std VGA's framebuffer instead of virtio-vga. 6.18's virtio-gpu has no suspend callbacks, so a guest that sleeps on it never gets its display back (#451). The second monitor of --second-head is a second output of virtio-vga, and is added only in the virtio-vga branch. So --second-head --s3 together would start a guest with one monitor, saying nothing about it.

No caller combines them today. 67 noted it in the read of #451's change.

What changed

The pair is refused at the start, beside --second-head's existing rule: "--second-head needs virtio-gpu, which --s3 gives up for std VGA's framebuffer".

How checked

bash -n. The rule uses the same [[ … ]] && die form as the line above it, which is safe under the script's set -e. Nothing in tools/image passes both flags, so no suite's run changes.