core: Replace oVirt logger with slf4j in bll

Replaces oVirt logger with slf4j in bll.

Stacktraces are logged only to debug for exceptions with clear custom
error message.

Change-Id: I82070b252197458422792c49b2135fcb9b9b1430
Bug-Url: https://bugzilla.redhat.com/1109871
Signed-off-by: Martin Perina <mperina@redhat.com>