conf: Add target type and model for pl011

conf: Add target type and model for pl011

We can finally introduce a specific target model for the pl011 device
used by mach-virt 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 pl011 is not
used for non-mach-virt guests and add a bunch of test cases.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=151292

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>