Apparently some devices do not comply with standards (there is no such
thing as USB version 2.01), but other than that works fine. So take into
account only the major number of bcdUSB field.
FixesQubesOS/qubes-issues#2158
Those devices are most likely attached using "PV USB" from another
domain, so it doesn't make sense to list them as available for further
passthrough.
QubesOS/qubes-issues#531
Even if particular PV USB implementation doesn't support it, still have
it included in QubesDB. It should be up to attaching code to decide.
Also, don't fail if xen-usbback module doesn't exist. This isn't the
only option (the other one is usbip over qrexec).
QubesOS/qubes-issues#531
