[Tails-dev] [review] slight doc improvement for virtualizati…

Hi,

I just tried to follow the documentation to use persistence in a VM
using virt-manager. It works well, although, I had preferred it to be
more precise that we need to use Tails Installer to install to the
virtual USB device. It could have been guessed, because it's mentioned
in the next sentence, but I think it's worth to add that.

I made a commit (90fb988f51444f52524b41239883cabf21267a02) to
451f:tails/doc/virtualizationdisk. Could somebody review this
eventually? Not urgent and maybe you think it's too much anyway. In that
case don't bother adding it.

cheers!
u.