core: fix connection resolving in update conn

Fix resolving connection attempt result returned from vdsm
in update storage server connection.
As part of the change, reuse the connection code in add/update storage
connection commands.

Change-Id: Ia16157d57af3619e32e20ede65521f16a6985b89
Signed-off-by: Alissa Bonas <abonas@redhat.com>