VirtIO now properly scans 8 MMIO slots, detects legacy vs modern mode,
uses correct virtqueue page alignment for legacy, and reads sectors.
Fixed disk image script to force FAT16 (mformat was creating FAT32).
init.elf successfully loaded from FAT16 disk via VirtIO on QEMU.
Adapter probes for VirtIO block device, reads init.elf from FAT16 disk,
parses ELF, maps into userspace, and launches. Falls back to embedded
minimal init if no disk is present.
Also: VirtIO MMIO driver, boot FAT16 reader, disk image build script,
Makefile targets for build/run/run-disk.
