The import dialog for KVM/libvirt based import is rather confusing. The problem is that you can specify username (and password) either as part of the URI or by filling the provided text fields. But these two methods are not interchangeable. Correct method depends on the transport protocol in use:

For SSH based imports you have to uncheck the 'Requires authentication' checkbox. You however still have to provide the user name (most likely 'root') as part of the URI, otherwise the connection will happen with user 'vdsm'. The existence of user name in URI is not checked and the behaviour is rather unexpected from user perspective. On the other hand specifying username/password in the provided text fields does not work; the username value is ignored and we don't support password-based logins.

For TCP/TLS it is the opposite. The user name provided in URI is ignored and username/password has to be provided via the text fields.

IMHO it would be better if the URI was constructed by  engine and import dialog would provide several fields to get the necessary info:

1) transport: ssh/tcp/tls
2) username
3) password
4) single text field for the path and extra parameters