xend: VBD QoS policy bits

Add the ability to define VBD QoS policy in the xend layer.

Consider the following vbd entry:

vbd = [
   'phy:/dev/server/virtualmachine1-disk,xvda1,w,credit=3D5000/s@50ms',
]

This means that a VM may perform 5000 I/O operations per second, with
credit being replenished every 50 milliseconds.

The 'credit' xenstore value is by the blkback driver to ratelimit I/O
operations for the specific device.

Signed-off-by: William Pitcock <nenolod@dereferenced.org>

tools/python/README.sxpcfg

@@ -82,6 +82,7 @@ vbd
   - uname
   - dev (ioemu:, .. etc)
   - mode (r, w. w!)
+  - credit
 
 vif
   - type