Shorter Oxford English Dictionary (that's the two volume one with 2,672 pages - not that short) gives it as virtuosi and virtuosos both being correct. But my impression is that the word has now been anglicised to the extent that virtuosos is now more common usage.
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Here's the youtube clip with the official video. Good fun stuff, not one of the great songs, but so what? Basic strumming is the right kin dof accompaniment, clever stuff would have spoiled it.

An dhere's a discussion about all this on a specialsit mandolin site.