blktap2: add blktap2 device class and device controller

blktap2: add blktap2 device class and device controller

blktap2 devices must be handled differently than blktap2
devices. blktap2 devices require a sysfs write to close the underlying
device, as well as extra sysfs writes when the domU is
paused/unpaused. The differences between blktap1 and blktap2 are great
enough to warrant the creation of a new device class, 'tap2', and
device controller for blktap2 devices.

  * add a new device controller (Blktap2Controller) and device class
    (tap2) for blktap2 devices
  * move blktap2 specific code from DevController to Blktap2Controller
  * if possible, check xenstore to determine block device class
  * use vmpath (/vm/<uuid>/) when releasing devices
  * modify linux hotplug cleanup script to handle blktap2 device
    removal

Signed-off-by: Ryan O'Connor <rjo@cs.ubc.ca>