storage: open secret driver connection at time of use

storage: open secret driver connection at time of use

Instead of passing around a virConnectPtr object, just open a connection
to the secret driver at time of use. Opening connections on demand will
be beneficial when the secret driver is in a separate daemon. It also
solves the problem that a number of callers just pass in a NULL
connection today which prevents secret lookup working at all.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>