fix(instances): the VM page shows each device's own settings

A NIC without QoS showed the previous NIC's limits, an empty CD-ROM the
previous drive's pool; a NIC without a source or model broke the page.
The boot order shows the target of block and network disks and of bridge
and direct NICs too. Refreshing the VM's pools before a snapshot or a
delete no longer fails on a disk outside every pool.

README.md

@@ -537,6 +537,7 @@ A rollback leaves the clone on a detached HEAD at the restored commit; the updat
 - A host's CPU graph shows the average since its previous point (normally 5 s) instead of a 1 s sample taken while a worker waits.
 - A VM's stats are polled every 5 s like the host graph, the first point as soon as the tab opens; each point is the average since the previous one, disk rates in MB/s (they were MiB/s labelled Mb/s); a counter the host does not report shows a gap instead of 0.
 - An ISO uploads to an SSH compute about 3.5 times faster (SFTP writes no longer wait for each 32 KiB); a failed write still fails the upload.
+- The VM page shows each NIC's own QoS and each CD-ROM's own pool (they showed the previous device's), and a NIC without a source or model no longer breaks it.
 - A running VM without a guest agent shows the IPv4 address the host learned by ARP (the lookup always failed before).
 - After a live migration the VM's NVRAM file is deleted on the source, when it is in libvirt's `/var/lib/libvirt/qemu/nvram` and the destination does not share that directory (a VM created there later with the same name would start with those UEFI variables).
 - gstfsd finds the guest's operating system the way libguestfs inspects it, so a root on LVM works, and it always answers (an error when it finds no or several operating systems). Update `/usr/local/bin/gstfsd` on every compute (see [Adding a Compute Node](#adding-a-compute-node), step 3).