Windows manages each sub-device of composite device as separate device -
each one requires separate call to CreateFile to obtain dev_handle.
Opening all sub-devices eagerly prevents other applications for
accessing any sub-device even if application that opened the device is
not using them.
This commit modifies WinUSB backend to call CreateFile lazily, so device
handle is opened when necessary (by claiming the interface) and release as
soon as possible (by releasing the interface).
Implementation details:
* winusbx_open becomes essentially no-operation
* winusbx_claim_interface ensures that device handle is opened
* interface used to initialize WinUSB handle has associated counter of
 claimed interfaces
* claiming and releasing interfaces modifies ref-counter accordingly
* when counter reaches zero on releasing interface, device handle is
 closed
Signed-off-by: Maciej Nowak <maciejt.nowak@gmail.com>