shell: Fix bad response when login with empty user name

Fix backend to return authentication failed if user is NULL.
Fix frontend to avoid empty user name.
Some refactoring.

Fixes #940
ws: Update test cases for previous commit

Closes #959
test: mvo.fedorapeople.org now uses https

The redirects were causing ./vm-download (and thus ./VERIFY)
to fail.

Closes #932
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
shell: Add input space validation when adding a server

Add input validation and visual style when error occurs.
Avoid resizing the dialog when error messages show up.
Remove get_focus and lose_focus functions.
Add some restrictions for empty host name in case "Next" button is enabled accidentally.

Fixes #943