Start passthrough VM and its dependents by device identity, not address

Adding a PCIe card can renumber the buses behind it, so a hostpci entry
that names a PCI address can pass through the wrong device. These start
the VM only after finding each device by vendor/device/subsystem ID or
by its MAC-pinned interface, repoint its resource mapping if it moved,
and refuse to start on anything ambiguous. Its dependents then wait for
a port on it to answer before starting, so they find their shares up.

systemd-system/dependents-start.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Start the guests that depend on the passthrough VM
+# If passthrough-vm-start.service fails this does not run, so nothing that needs
+# the VM is started without it.
+Requires=passthrough-vm-start.service
+After=passthrough-vm-start.service pve-guests.service
+
+[Service]
+Type=oneshot
+RemainAfterExit=true
+ExecStart=/opt/ergosteur/start-dependents.sh
+# Covers WAIT_SECS in conf/dependents.conf plus the starts themselves.
+TimeoutStartSec=900
+
+[Install]
+WantedBy=multi-user.target