core: Replace oVirt logger with slf4j in bll/network

Replaces oVirt logger with slf4j in bll/network package.

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

Change-Id: I9af76a3f79de2677060eb1dc36420f6be6433670
Bug-Url: https://bugzilla.redhat.com/1109871
Signed-off-by: Martin Perina <mperina@redhat.com>