libdlogutil: put logs into vector before polling

libdlogutil: put logs into vector before polling

Eventually we want the API to return a single log. With poll at the
front, it would be done for every log returned, which is unnecessary.

Additionally, remove the "poll returned nothing" condition which
allowed all buffers to be drained at the same time. It shouldn't
be helping much in practice and can be simplified away.

The callback is temporarily not allowed to return 1. This will go
away in the final version of the API.

Change-Id: I4ccb0a613a902484303f651830a946ccc8718130
Signed-off-by: Michal Bloch <m.bloch@samsung.com>