qemu_domain: Strip <acpi/> from s390(x) definitions

qemu_domain: Strip <acpi/> from s390(x) definitions

The s390(x) machines never supported ACPI. That didn't stop users
enabling ACPI in their config. As of libvirt-9.2 (98c4e3d073) with new
enough qemu we reject configs which require ACPI, but qemu can't satisfy
it.

This breaks migration of existing VMs with the old wrong configs to new
libvirt installations.

To address this introduce a post-parse fixup removing the ACPI flag
specifically for s390 machines which do enable it in the definition.

The advantage of doing it in post-parse, rather than simply relaxing the
ABI stability check to allow users providing an fixed XML when migrating
(allowing change of the ACPI flag for s390 in ABI stability check, as it
 doesn't impact ABI), is that only the destination installation needs to
be patched in order to preserve migration.

To mitigate the disadvantage of simply stripping it from all s390(x)
configs the hack is not applied when defining or starting a new domain
from the XML, to preserve the error about unsupported configuration.

Resolves: https://issues.redhat.com/browse/RHEL-49516
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>