Update QEMU machine type for all our VMs
Most currently are
<type arch='x86_64' machine='pc-1.1'>hvm</type>,
some simply because we created them a while ago, some because our
virt-install command line has not been updated for ages. On my current
sid (== almost Stretch these days), virt-manager creates
machines when instructed to create a Stretch VM. Recent discussions on the libvir-list ML suggest that q35 is the way to go nowadays. Updating this might improve performance.