refactor(instances): one way to reach a VM, its console and its devices

The .vv console file is built from the VM found by its UUID, with the
console page's getters, instead of looking the VM up by name five times;
a console without a listen address or password now gives 127.0.0.1 and
an empty password instead of "None". Removed methods nothing calls: the
by-name getters and power methods of wvmInstances, managed save, the vCPU
mode getter (it read the wrong attribute), the old NIC editor and its
bridge lookup. CD-ROM media and NIC hotplug use _definitions() like the
other device edits, the disk editor takes the bus prefix from
util.vol_dev_type, and an unreadable disk is logged, not printed.

README.md

@@ -540,6 +540,7 @@ A rollback leaves the clone on a detached HEAD at the restored commit; the updat
 - 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 volume of an LVM or iSCSI pool can be attached to a VM, and bridge and direct NICs can be boot devices. The NIC model `default` lets libvirt choose, and `rt18139` is `rtl8139`: a VM with either did not start; a migration corrects the default NIC type setting.
 - Cloning a UEFI VM no longer defines a storage pool on the host's NVRAM directory; the clone gets its own copy of the VM's UEFI variables.
+- The console .vv file finds the VM by its UUID; without a listen address or password it gives 127.0.0.1 and an empty password instead of "None".
 - 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).