Wait for the devices configured at start-up, before allowing the boot to

Wait for the devices configured at start-up, before allowing the boot to
proceed.  This avoids the race (bug #549) that would result in the infamous
VFS: Cannot open root device "sda1" or unknown-block(0,0) message even when
the hotplug layer had succeeded to configure the device.

Note that The Infamous Message has many possible root causes -- this change
only fixes one of them.  This particular root cause is characterised by
successful boots interleaved with unsuccessful ones, and diagnose.py showing
both front and backend drivers in the Connected state.

Closes bug #549.

Signed-off-by: Ewan Mellor <ewan@xensource.com>