Hi Stefano,
I've re-written STR-19 (https://projects.linaro.org/browse/STR-19) now I have a hopefully better understanding of the relationship between IOREQ backends and how QEMU is launched with Xen. I see this approach as a stop-gap for testing additional virtio devices. Eventually we need to specify a new card for a Xen aware vhost-user launching stub that will link our hypervisor agnostic user-space daemons to the Xen specific virtio instance. I think we can hold off on that until we have a better idea of the sort of interface Xen and other type-1's need to provide for them.
I've also raised STR-20 (https://projects.linaro.org/browse/STR-20) to cover the work to fix up the current build and allow for using a leaner native qemu-system-aarch64 to be used instead of qemu-system-i386 and it's additional baggage. I have patches in train for that which I'll post soon.
Could you have a look over both for any obvious snafus?