xend: VBD QoS policy bits

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>