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=1511421Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>