perf(qemu): match `amd64` speeds on `arm64`

I've done a few tweaks to the Labgrid configuration for the arm64 QEMU toolchain by following the example in the QEMU Introduction page.

The pauth-impdef=on CPU feature has given the most significant performance improvement, cutting file transfer speeds by 5 times, and bringing them in line with the ones seen on amd64.

These improvements should be enough to close #11 (closed).