ovirt-foreign-menu: Only set domain_valid once
    
    In the case of having a valid storage domain without any ISO files, this
    variable can be reset to FALSE again in the next iteration of the loop,
    resulting in a misleading error message presented to the user.
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>
    Acked-by: Victor Toso <victortoso@redhat.com>