The information about connected devices is obtained from a ROM file named
'usb_devices', which is supposed to contain a device list as in the device
report generated by the USB driver (see issue #1506).
A policy for 'report_rom' would look like:
<policy label="vbox -> usb_devices" report="usb_drv -> devices"/>
If the 'usb_devices' ROM file is not available, a warning message gets
printed and VirtualBox continues without USB pass-through support.
The devices to be passed-through need to have a matching device filter in
the '.vbox' file. Example:
<USB>
<DeviceFilters>
<DeviceFilter name="USB Scanner" active="true" vendorId="04a9"
productId="2220" remote="0"/>
</DeviceFilters>
</USB>
The feature was tested with HID devices (mouse, keyboard) and a flatbed
scanner. Mass storage devices didn't work correctly (they also didn't work
with VirtualBox on Linux without the closed-source extension pack).
It should be made sure that the USB driver does not try to control the
devices to be passed-through itself, for example, when passing-through
a HID device, the '<hid/>' config option should not be set.
Fixes#1507
