tools: usb: usbip: Update README

Update README file:
- remove outdated parts
- clarify terminology and general structure
- add some description of vUDC

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>