Add QEMU Tweak setting to turn off the boot delay

UTM adds the QEMU argument `-boot menu=on` that makes BIOS guests show the "Press ESC for boot menu" prompt with a three second timeout. UEFI guests also have a timeout while waiting for ESC key press, but this is baked into the firmware if I understand correctly.

It would be great if I could avoid this unnecessary delay when starting my VM. I would suggest a "Prompt for boot menu (ESC)" tweak to be added to the QEMU settings. It would be enabled by default. If disabled:

- BIOS VMs get `-boot menu=off` parameter
- UEFI VMs use alternate firmware file that doesn't have delay baked in