core, webadmin: allow edit unattached storage conn

Allow editing (updating) the path (connection)
of an unattached storage domain.
Before this change, it was allowed only for domains in maintenance
state.
Fix error messages accordingly to reflect this change as well.

Change-Id: I42185d9306f5242b2258adbf8072288184393b63
Signed-off-by: Alissa Bonas <abonas@redhat.com>