Add support for booting from file-backed disk

[struanb]

Currently RunCVM boots from a virtiofs-mounted root filesystems, which is highly flexible and space-efficient and reasonably performant, but not as performant as a file-backed disk.

RunCVM supports mounting file-backed disks but not for the root filesystem.

If feasible, provide an option to enable this.

[struanb]

Worth noting that the latest RUNCVM_DISKS environment variable behaviour, which mounts a file-backed virtual disk populated with any pre-existing image contents at almost any desired filesystem location (except, currently, /) will provide much better VM disk performance in a range of scenarios where these can be effectively used.

For example, if the booted system is itself primarily running applications within Docker, mounting a single virtual disk at /var/lib/docker should be sufficient to deliver high disk performance.