Add NewConnectWithAuthDefault
This PR adds a new method to be able to make use of the virConnectOpenAuthDefault callbacks. Even if it has an integration test, it's currently failing because it asks for the username even if it's present in the libvirt connection URI:
=== RUN TestConnectionWithAuthDefault
Please enter your authentication name: integration_test.go:294: virError(Code=45, Domain=7, Message='authentication failed: Failed to start SASL negotiation: -1 (SASL(-1): generic failure: All-whitespace username.)')