Implement usb-auto-passthrough policy

Check the usb-auto-passthrough VM property. If not allowed (set to
false), retain USB device in Dom0.

OXT-64

Signed-off-by: Kevin Pearson <kevin.pearson.4@us.af.mil>
Merge pull request #2 from pearsonk/master

[v2] Implement usb-auto-passthrough policy