virt-install and virt-manager both default to explicitly setting
"io='native'" in the disk "driver" tag. virsh, however, does not and also
does not provide an option to specify that setting at all. As a result,
disks use a different IO mechanism (the default, "threads") when attached
post-setup using virsh. Adding this option allows users to keep disk
performance consistent for disks attached at install, and those attached
afterward.