This patch implements the new extended allocation scheme on the blkfront end.
Note that there is a new xenbus node for the extended stuff, called
"virtual-device-ext".  On a blkfront_probe(), if nothing is found in
"virtual-device", then it will go looking in "virtual-device-ext".  This
prevents old guest kernels from potentially mis-interpreting data in
"virtual-device" (and then subsequently crashing).

Signed-off-by: Chris Lalancette <clalance@redhat.com>