Features:
+* enumerate virtiofs devices during boot-up in a generator, and synthesize
+ mounts for rootfs, /usr/, /home/, /srv/ and some others from it, depending on
+ the "tag".
+
+* automatically mount one virtiofs during early boot phase to /run/host/,
+ similar to how we do that for nspawn, based on some clear tag.
+
* make systemd-pcrphase accessible via varlink so that clients can measure
things reasonably freely, and get a proper entry in the TPM event log we
maintain, correctly synchronized