multipathd is not starting waitevent checker for single paths

multipathd is not starting waitevent checker for single paths

After multipathd was started, any SCSI disks that would be added afterwards
would not trigger multipathd to create a waitevent thread.

The waitevent thread listens for kernel's offline/online events and thoroughly
checks what the kernel sees with what multipathd thinks and if something is
off,
whacks multipathd to the right state.

For devices which did not have a kernel device mapper helper (hp_sw, rdac,
etc) and only have one single path, when the link experiences a momentary blib
with I/O on it the path would be marked as failed _only_ by the kernel. This
event
would _not_ be propagated to multipathd (b/c it did not have a waitevent thread
create). Multipathd would only do the path checker which would provide a
PATH_UP event (rightly so - as the path would only be down for a second or so).
However, the device mapper path group would be marked as failed, and any
incoming I/O would be blocked (if queue_if_no_path was set) or fail.

The end result was the multipathd would think everything was peachy while the
kernel would be failing (or queueing) the I/O to the multipath device.

References: bnc#473841

Signed-off-by: Hannes Reinecke <hare@suse.de>