tests/hetzner: Organize VM tests into subtests.

This should make the test report look way less messy and it should
continue with other subtests even if an out of memory condition occurs.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nix/hetzner: Fix name collisions of swap parts.

The swap partitions of the default partitioning layout both have the
same name, so we get a warning showing that "swap" is already assigned.
That's why they're now called swap1 and swap2.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nix/hetzner: Limit clearpart to sda and sdb only.

If we have a system with more disks than the default two, let's just not
touch those disks and only work on the first two disks. Should the user
really want to erase/format more disks, he or she could do so by just
setting the partitions attribute accordingly with their own partitioning
layout.

Also, this significantly speeds up partitioning.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
tests/hetzner: Use -boot once instead of eject.

We don't need to eject any virtual drive here, because QEMU already has
a way to boot a device just for the first boot. Exactly what we need for
the rescue system.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>