staging: usbip: userspace: usbip_unbind.c: implement using libsysfs

Modify unbind to use libsysfs, and include a check to verify that the
device is actually using usbip-host before proceeding. The output
messages have been changed to be consistent with `usbip bind'.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>