Remove logging in libvirt _connect_auth_cb to avoid eventlet locking

The _connect_auth_cb callback is logging a warning if credentials are
passed to the method from libvirt when connecting, but the comment right
above where the connection method is called on libvirt, it says don't do
any logging in that function because it can cause issues with how
eventlet does logging.

This has probably never been an issue because libvirt hasn't passed
credentials back to the callback, but if it did at some point we could
have a nasty locking problem so let's clean it up now.

See commit e4b0d8e9 for some related history.

The nested method is moved up a level so it can be tested directly.

Change-Id: Ie47aabbf81c5aeda9e12e1c9acc11f6c70c4bea7
Merge "Remove logging in libvirt _connect_auth_cb to avoid eventlet locking"