Support ISOHybrid USB mass storage devices

[InvoxiPlayGames]

This needs some amount of testing before merging.

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Expected behaviour:

• Burned ISO9660 discs still work as they do in upstream XeLL
• ISOHybrid ISO9660 images built with grub-mkrescue or similar do work when imaged to a USB storage device with dd or similar (e.g. ArchPOWER's Xenon ISO)

Caveats:

• Regular ISO9660 discs still do not work when imaged to a USB storage device. This might be able to be changed in the future, but this behaviour should not be encouraged.
• Certain other forms of ISOHybrid also do not work. That behaviour, too, should not be encouraged.

Have I mentioned that ISOHybrid is a terrible hack yet? It's a terrible hack. I don't like it. I don't think it should be used, not on current post-2010s IBM PC compatibles nor on embedded devices like the Xenon.