ovirt-foreign-menu: Fetch ISO names using GTask API
    
    Similar to the previous commit, the ISO dialog will fetch the result
    asynchronously, rather than relying on the "notify::files" signal from
    OvirtForeignMenu object. It also enables error to be shown if anything
    goes wrong.
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>