Implement usb-auto-passthrough policy enforcement

If a USB device is plugged in and no always-assign policy is set, check
the policies (usb-enabled and new usb-auto-passthrough) to determine if
it is allowable to automatically pass the new device to the VM with the
current focus. If not, keep the device with Dom0.

OXT-64

Signed-off-by: Kevin Pearson <kevin.pearson.4@us.af.mil>