In early userspace (initramfs), mount a VM image as the new root and let
the boot process switch_root to it.
You can package qemu-nbd (from qemu), mount.ntfs (from ntfs-3g),
losetup (from util-linux) and various tools into the initramfs
for your mounting needs, so theoretically any format of root filesystem
images can be used.
- You must manually keep the host kernel and VM image kernel version in sync. Otherwise, kernel modules will fail to be found, resulting in missing drivers.