inventory-qcow2: Use physical cores, match libvirt CPU invocation
I was investigating some rpm-ostree CI performance issues in
https://github.com/projectatomic/rpm-ostree/pull/1362
and I discovered that my previous change to use `-smp $cpus` was
suboptimal. See the linked libvirt discussion; basically we
really want to use physical cores, otherwise the guest kernel
is going to make bad scheduling decisions.
We also take care to also minimize that set by the number of
CPUs available to us; we may be running in a container with
a cgroup for example.