ovirt-foreign-menu: New function storage_domain_validate()
    
    It may be useful to know why the storage domain has not been listed,
    given that there are different reasons for that. To make it easier to
    provide more detailed debug messages, we move code from the callback
    function to this new one.
    
    Acked-by: Christophe Fergeau <cfergeau@redhat.com>
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>