qemu: fix default machine for argv -> xml convertor

qemu: fix default machine for argv -> xml convertor

Historically the argv -> xml convertor wanted the same default machine
as we'd set when parsing xml. The latter has now changed, however, to
use a default defined by libvirt. The former needs fixing to again
honour the default QEMU machine.

This exposed a bug in handling for the aarch64 target, as QEMU does not
define any default machine. Thus we should not having been accepting
argv without a -machine provided.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>