Document that Tails requires I/O APIC being enabled in the VirtualBox VM config

The follow-ups to various bug reports about regressions against Tails 2.7 tell us that enabling I/O APIC in the guest’s motherboard settings fixes the regression many people have reported.

Side-note: I’ve also seen recommandations to switch the VM to 64-bit. I trust it helps (apparently it somehow enables I/O APIC) but that’s sub-optimal since Tails 2.x only support the guest utilities in 32-bit VMs (#11848 (closed)).

Feature Branch: doc/12001-virtualbox-needs-io-apic

Related issues

Related to #11848 (closed)
Related to #11965 (closed)

Original created by @intrigeri on 12001 (Redmine)

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