Thanks to this code merged into the Virglrenderer and this now-merged Mesa code , there is support for AV1 video decoding that can be enjoyed by QEMU/KVM guests -- assuming the host GPU is capable of AV1 decode.
Feng Jiang with China's Kylin OS project worked on this AV1 decode hook-up and shared this screenshot of the Blender film Big Buck Bunny running successfully with AV1 decode inside a QEMU/KVM guest:
Mesa 23.3 will be out in Q4 as the next major feature series for this open-source GPU driver stack.