Support direct kernel booting a disk image

Support direct kernel booting a disk image

This requires two things:
1. We need to generate a split initrd again to pass to -initrd
2. We need to synthesize a root= argument as we can't rely on
gpt-auto-generator since we're not using EFI.

We move the partition and root= specific stuff to a new file
partition.py so we can access it from qemu.py as well.

We also introduce extract_pe_section() since we now use the logic
twice.