Start qemu uefi using pflash instead of bios

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 I'm not sure whether the issue was an outdated ovmf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