Add a DriveDetach() method to properly power down USB hard disk enclosures

Add a DriveDetach() method to properly power down USB hard disk enclosures

This work is inspired by this thread

 http://thread.gmane.org/gmane.linux.hotplug.devel/14079

that is referencing this blog entry

 http://elliotli.blogspot.com/2009/01/safely-remove-usb-hard-drive-in-linux.html

The only difference here is that instead of unbinding the usb driver
from the usb device, we unbind the usb-storage driver from the usb
interface in question. This is to better handle multi-function devices
(e.g. multiple USB interfaces).

Things like GVfs (ie. Nautilus) can use this new method to offer an
"Eject" option for USB enclosures that can invoke DriveDetach() on
this service. (In GVfs, such devices currently have no eject option
since they don't support removable media).