Remove virtio legacy transport support
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.