perf(instances): the VM stats no longer wait a second per poll

Each stats answer carries a signed sample of the counters, bound to the VM,
its compute and UUID. The next poll sends it back and the rates are the
averages since then; a sample from another run of the VM (domain ID),
another host CPU count or device list, or older than 30 s falls back to
one second as before. Windows are measured on the monotonic clock.

Now that a poll is cheap, the Stats tab polls every 5 s like the host
graph, the first time as soon as it opens (pollWhileVisible takes the
delay of the first request).

A counter the host reports as unsupported (-1) or that went back gives no
value instead of 0 or a jump; a NIC without a target keeps its chart, a
pool volume disk is measured, and a disk or NIC attached after the page
loaded no longer stops the other charts. Disk rates are MB/s (they were
MiB/s labelled Mb/s), network Mbit/s; the CPU tooltip names the window.

README.md

@@ -535,6 +535,7 @@ A rollback leaves the clone on a detached HEAD at the restored commit; the updat
 - The log rotates (10 MB × 5) in `data/webvirtcloud.log`.
 - A disk can be grown while its VM runs or is paused, not only when it is shut off.
 - 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.
 - 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).