qemu: ensure default machine types don't change if QEMU changes
It is increasingly likely that some distro is going to change the default "x86" machine type in QEMU from "pc" to "q35". This will certainly break existing applications which write their XML on the assumption that it is using a "pc" machine by default. For example they'll lack a IDE CDROM and get PCIe instead of PCI which changes the topology radically.
Libvirt promises to isolate applications from hypervisor changes that may cause incompatibilities, so we must ensure that we always use the "pc" machine type if it is available. Only use QEMU's own reported default machine type if "pc" does not exist.
This issue is not x86-only, other arches are liable to change their default machine, while some arches don't report any default at all causing libvirt to pick the first machine in the list. Thus to guarantee stability to applications, declare a preferred default machine for all architectures we currently support with QEMU.
Note this change assumes there will always be a "pc" alias as long as a versioned "pc-XXX" machine type exists. If QEMU were to ship a "pc-XXX" machine type but not provide the "pc" alias, it is too hard to decide which to default so. Versioned machine types are supposed to be considered opaque strings, so we can't apply any sensible ordering ourselves and QEMU isn't reporting the list of machines in any sensible ordering itself.
Reviewed-by: Andrea Bolognani <abologna redhat com> Signed-off-by: Daniel P. Berrangé <berrange redhat com>
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