This RFC comes from the recent discussion on buffer sharing [1],
specifically about the need to share resources between different
virtio devices. For a concrete use case, this can be used to share
virtio-gpu allocated buffers with the recently proposed virtio video
device [2], without the need to memcpy decoded frames through the
guest.

[1] https://markmail.org/thread/jeh5xjjxvylyrbur
[2] https://markmail.org/thread/yb25fim2dqfuktgf