For pool which relies on remote resources, such as a "iscsi" type
pool, since how long it takes to export the corresponding devices
to host's sysfs is really depended, it could depend on the network
connection, it also could depend on the host's udev procedures. So
it's likely that the volumes are not able to be detected during pool
starting process, polling the sysfs doesn't work, since we don't
know how much time is best for the polling, and even worse, the
volumes could still be not detected or partly not detected even after
the polling.  So we end up with a documentation to prompt the fact,
in virsh manual.

And as a small improvement, let's explicitly say no LUNs found in
the debug log in that case.