Transitional virtio devices (which have both legacy and virtio 1.0
support) have been the default since QEMU 2.7.0, which was released
8 years ago. Basically every non-commercial Linux distro with an older
QEMU has been EOL already.
The legacy transport is also one of the few places where port I/O is
necessary, which is non-trivial to sandbox even once we have IOMMU
support. As it isn't possible to distinguish legacy and modern virtio
devices for pcid, this would mean that all virtio drivers have to be
started as privileged even if it turns out the modern transport is
supported by the VMM.
bjorn3