Document when/how to get early boot messages

[cfergeau]

Apple's virtualization framework only implements serial logging over virtio, which makes it hard to get early boot messages.
Part of this is caused by the fact that virtio-serial in the guest is a module, and is only loaded late in the boot process through udev.

However, what can be seen seems to depend on more factors than this:

• image being booted (fedora coreos VS fedora cloud)
• boot method (efi VS direct kernel boot)
• kernel parameters
• use of a virtio-serial device VS use of --gui

It would be nice to document all of this once and for all, in particular if there's a non-default configuration which allows to get more logs out of a failed boot.

[cfergeau]

This came up again recently in coreos/fedora-coreos-docs#711 (comment)