supervdsm: some log beautification

- log return value, not only function call
- keep try-block cleaner

Patchset-1 Initial implementation.
Patchset-2 Used @wraps decoration from functools.

Change-Id: I583ea9549481196e82da174cc3193786239ff19d
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17331
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>