Automatically download prebuilt OVMF files

The code for trying to find where the OS has installed OVMF files has been
removed. You can still pass your own OVMF files in via command-line or env vars,
but by default the code will download prebuilt OVMF files from
https://github.com/rust-osdev/ovmf-prebuilt.

Author: nicholasbishop