xen-blkback: do not use state 3 (XenbusStateInitialised)

Linux will not connect to a backend that's in state 3
(XenbusStateInitialised), it needs to be in state 2
(XenbusStateInitWait) for Linux to attempt to connect to the backend.

The protocol seems to suggest that the backend should indeed wait in
state 2 for the frontend to connect, which makes state 3 unusable for
disk backends.

Also make sure blkback will connect to the frontend if the frontend
reaches state 3 (XenbusStateInitialised) before blkback has processed
the results from the hotplug script (Submitted by Nathan Friess).

MFC after: 1 week