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>