conf: Add target type and model for spapr-vty
    
    We can finally introduce a specific target model for the spapr-vty
    device used by pSeries guests, which means isa-serial will no longer
    show up to confuse users.
    
    We make sure migration works in both directions by interpreting the
    isa-serial target type, or the lack of target type, appropriately
    when parsing the guest XML, and skipping the newly-introduced type
    when formatting if for migration. We also verify that spapr-vty is
    not used for non-pSeries guests and add a bunch of test cases.
    
    This commit is best viewed with 'git show -w'.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1511421
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>