piix: fix xenfv regression, add compat machine xenfv-4.2

piix: fix xenfv regression, add compat machine xenfv-4.2

With QEMU 4.0 an incompatible change was added to pc_piix, which makes it
practical impossible to migrate domUs started with qemu2 or qemu3 to
newer qemu versions. Commit 7fccf2a06890e3bc3b30e29827ad3fb93fe88fea
added and enabled a new member "smbus_no_migration_support". In commit
4ab2f2a8aabfea95cc53c64e13b3f67960b27fdf the vmstate_acpi got new
elements, which are conditionally filled. As a result, an incoming
migration expected smbus related data unless smbus migration was
disabled for a given MachineClass. Since first commit forgot to handle
'xenfv', domUs started with QEMU 4.x are incompatible with their QEMU
siblings.

Using other existing machine types, such as 'pc-i440fx-3.1', is not
possible because 'xenfv' creates the 'xen-platform' PCI device at
00:02.0, while all other variants to run a domU would create it at
00:04.0.

To cover both the existing and the broken case of 'xenfv' in a single
qemu binary, a new compatibility variant of 'xenfv-4.2' must be added
which targets domUs started with qemu 4.2. The existing 'xenfv' restores
compatibility of QEMU 5.x with qemu 3.1.

Host admins who started domUs with QEMU 4.x (preferrable QEMU 4.2)
have to use a wrapper script which appends '-machine xenfv-4.2' to
the device-model command line.  This is only required if there is no
maintenance window which allows to temporary shutdown the domU and
restart it with a fixed device-model.

The wrapper script is as simple as this:
  #!/bin/sh
  exec /usr/bin/qemu-system-i386 "$@" -machine xenfv-4.2

With xl this script will be enabled with device_model_override=, see
xl.cfg(5). To live migrate a domU, adjust the existing domU.cfg and pass
it to xl migrate or xl save/restore:
  xl migrate -C new-domU.cfg domU remote-host
  xl save domU CheckpointFile new-domU.cfg
  xl restore new-domU.cfg CheckpointFile

With libvirt this script will be enabled with the <emulator> element in
domU.xml. Use 'virsh edit' prior 'virsh migrate' to replace the existing
<emulator> element to point it to the wrapper script.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Message-Id: <20200327151841.13877-1-olaf@aepfle.de>
[Adjust tests for blacklisted machine types, simplifying the one in
 qom-test. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>