usbip: vhci-hcd: Add vhci struct

In order to support SuperSpeed devices, a USB3 HCD is added to
share the USB2 HCD. As a result, a VHCI is composed of two
vhci_hcds associated with the two HCDs respectively. So we add
another level of abstraction, vhci, and thus this vhci structure.

Signed-off-by: Yuyang Du <yuyang.du@intel.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>