Added usb-auto-passthrough VM property

Added the boolean property 'usb-auto-passthrough' to the xenmgr_vm
interfaces:
  com.citrix.xenclient.xenmgr.vm
  com.citrix.xenclient.xenmgr.vm.unrestricted

OXT-64

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