elog_watch: Fix parsing of elog add requests

Requests come in the form "oa{sa{sv}}". However, the way sdbusplus was
interpreting the type of our message "a{oa{sa{sv}}}" since tuples are
not allowed to consume multiple arguments during the read call as that
would be ambiguous. This fixes the type issues.

Prior to the change to sdbusplus that introduces error handling for the
read calls, the sd_bus_message_{enter,exit}_container were failing
during the read on the pair. Luckily this produces the expected result
for the read and our old code was "working".

This also cleans up an unnecessary string move.

Tested:
    Builds and no longer produces errors on zaius when elogs are added.

Change-Id: Ifc5394f3f361e8932c939376bd0bf5b4e3ca589c
Signed-off-by: William A. Kennington III <wak@google.com>