Support bringing up block device as rootfs

[arihant2math]

• Adds the concept of disks that can be mounted and unmounted (exposed in /dev).
• Adds support for virtio disks drivers. This allows for bringing up large filesystems.
• To allow for pivot_root to function this also required the implementation of pivot_root and unmount.

Running the following commands will mount the disk and switch to ubuntu:

mkdir /new-root
mount -t ext4 /dev/vda /new-root

# Create a mount point for the old root inside the new root
mkdir -p /new-root/old-root

cd /new-root

# Mount devfs in the new root so that console is available
mkdir -p dev
mount -t devfs devfs dev

pivot_root . old-root

# change to ubuntu
exec chroot . /bin/bash <dev/console >dev/console 2>&1

Future work:

• intramfs?
• Allow for directly loading from a disk.
• Ubuntu's /bin/ls has some weird crashing issue involving segfaults and /bin/bash has issues with stack smashing.