Simplify booting from block devices

[nohajc]

I had an idea how to simplify booting from block devices. Since init.krun is a virtual file implemented in the virtiofs passthrough module, we'd normally have to manually copy it to the device from which we want to boot and also adjust kernel cmdline to override the default root settings.

Instead, if krun_set_root() was never called but there is a block device, we can still create a dummy rootfs just for executing init and then switch to the real root with a similar mechanism initramfs would use.

This relies on setting KRUN_BLOCK_ROOT environment variable (internally) as well as reading /proc/filesystems so we can implement the behavior of mount -t auto for the real root.

I also added another custom ioctl which I use for cleaning up the temporary root when no longer needed.

[nohajc]

Hmm, just realized /dev/vda might not be the best default and we'll need a way to configure it anyway. Also @ggoodman is working on something similar (#346).

[mtjhrc]

Really cool idea! I'm not sure if we want this enabled implicitly though. Perhaps you could add another API function, something like krun_set_root_disk_remount(ctx_id, "/dev/vda") that enables this functionality and also allows for configuring the disk?

[nohajc]

Sounds good. Perhaps the function could also take the filesystem type (with the auto type accepted too) and other mount options?

[mtjhrc]

Sounds good. Perhaps the function could also take the filesystem type (with the auto type accepted too) and other mount options?

Sure, sounds good.