Tests will now fail with a timeout error after 2 minutes. That should
bee more than enough to run any test.

This also switches to the new API of the uefi crate that was
introduced in version 0.31. The old api was deprecated in 0.32 and
will be removed in 0.33.
See https://github.com/rust-osdev/uefi-rs/blob/ac19526656953c32e8e0ef7bc703f7700b151ae4/docs/funcs_migration.md
and rust-osdev/uefi-rs#893

Switch from using the "-bios" flag to using "-pflash" instead.
Using "-bios" is not recomended see:
https://lists.katacontainers.io/pipermail/kata-dev/2021-January/001650.html

This also allows us to easily update the ovmf-prebuild binaries.

This fixes rust-osdev#400. I'm not sure whether the issue was an outdated ovmf
version or the bios-pflash change. I can't tell what version is
published with the ovmf-prebuild crate, so I can't check.

Ovmf binaries are no longer provided by the ovmf-prebuild crate.
Instead we now download them from the ovmf-prebuild github releases of
the same crate. Unlike the crate, those are actually updated.

Fixes rust-osdev#400