core: improve messages of create/delete snapshot

Improve text of event log messages for creation and deletion
of snapshots (and live snapshots).

Change-Id: I29bc8638dc8506bb09efd00bb6f96b1ba8a0a825
Bug-Url: https://bugzilla.redhat.com/896418
Signed-off-by: Alissa Bonas <abonas@redhat.com>