Comment 2 for bug 1780138

To expand on comment#1, from libvirt v4.7.0 onwards, thanks to this
libvirt commit[*] libvirt will ensure that the default machine type for
"x86" will keep as 'pc', *if* it is available. Then the assumption
(that "guest.os_mach_type is None" means, then you have 'pc' machine
type) we have today Nova's libvirt driver will hold. However, relying
on such assumptions is still not good.

Nova needs to explicitly configure a machine type for "x86"---either by
using libosinfo (/me needs to learn how to use this API) which
recommends machine type based on what is supported by the QEMU on your
host; or by getting the default machine type from libvirt (from its
getCapabilities() API).

[*] https://libvirt.org/git/?p=libvirt.git;a=commit;h=26cfb1a3 -- "qemu:
ensure default machine types don't change if QEMU changes"