Document how to boot a VM into rescue mode for troubleshooting

[afbjorklund]

Description

It is possible to boot into single user mode with qemu, at the moment.

see #974 (comment)

It should be possible to do this with vz, but it is not yet implemented...

https://pkg.go.dev/github.com/Code-Hex/vz/v3#LinuxBootLoader

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We might also want to add some details on how to mount a disk image.

• Is there a way to run fsck when boot failed? #2104

And to add a section on how to use the serial.sock and friends (e.g. socat)

Most of this is only for advanced users and for troubleshooting problems