rbd-wnbd: wait for disks to become available

rbd-wnbd: wait for disks to become available

After a WNBD mapping is created, the driver informs Storport that
the bus changed, expecting it to rescan the bus and expose the disk.
This is an asynchronous process and it usually takes a matter of
milliseconds, up to a few seconds under significant load.

The fsx librbd test ocasionally fails to open the disk as it isn't
ready yet. Unlike the Python rbd-wnbd test, it doesn't perform
any polling.

For convenience, WNBD now includes a helper function called
WnbdPollDiskNumber. We're going to use it to wait for the new
disk attachments to become available.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>