It can happen during device reconfiguration - do not decide to expose
the device until its known what device it will be.
This fixes bug where root.img was visible in qvm-block as normal device
and could be detached.
xenstored does handle concurrent writes very harshly - it aborts the
whole transaction if any other write happened the same time. For udev
(which process all the events almost at once) it means hundreds of
retries and in some times even exceed udev timeout (60s or so).
To prevent this problem, add locking to allow only one such event being
processed at once. It looks like it should slow down the system startup,
but actually it does otherwise.
