feat(instances): a new VM is made on one form in three steps

The create page had three tabs (flavor, custom, template), each with its
own copy of the same create form; every flavor rendered one more copy in
a dialog, and fixes had to be made in each copy. All of them posted the
same form to the same job, differing only in where the disk comes from.

The page is now one form in sections: name and size, disk, networks and
an Advanced section. The disk section offers three sources as radio
cards (a new disk, existing volumes, a copy of a template); only the
chosen source's fields are shown, and the others are disabled, so they
are neither sent nor validated. A template's copy goes into the
template's pool unless another is picked. Flavors are presets that fill
vCPU, memory and disk size and pick a new disk; they are added and
deleted in a dialog outside the form. Every source uses the
per-interface network rows, so a new disk no longer requires a MAC:
libvirt gives an interface without one its address, as it did for the
other sources. A refused form comes back with its source, volumes,
template and networks, and the focus on its first error. Without a
storage pool or a network the page shows why instead of the form, and a
host that cannot be read is a message, not a 500.

The host dialog and the machine page share the form's look: hosts are
rows with their CPU and memory use, the machine page is a card with
Cancel/Next and the XML way is a collapsible section. All three steps
show the same step indicator, done steps linking back.

The create job refuses more than one disk source (the page sends one; a
crafted request could send two, and the job would have used the first).
A field with help text is described by its help and its error.

Frontend review fixes: the presets and the disclosure buttons show a
focus ring for keyboard users; a disclosure spans its card and its
chevron turns while open; a preset says it picks a new disk; numeric
fields bring a numeric keyboard and MAC fields no autocorrection; the
machine page's Next waits for a machine type. Tom Select's control
keeps the text colour of the select it replaces: its own fixed colour
made typed text invisible on dark Bootswatch themes, on every page that
uses it.

README.md

@@ -583,6 +583,7 @@ A version from before v3.0.0 has no worker: `systemctl disable --now webvirt-wor
 - A disk can be grown while its VM runs or is paused, not only when it is shut off. A disk on an LVM pool cannot be grown (libvirt cannot resize its volumes): the resize says so instead of failing, and the VM's other disks still grow.
 - Enabling vCPU hot plug keeps the VM's current vCPU count (it enabled all of them up to the maximum).
 - 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 new VM is made in three steps (host, machine, details) on one form: the disk is a new one, existing volumes or a copy of a template, and flavors are presets that fill vCPU, memory and disk size. A request with more than one disk source is refused.
 - The Logs page records who added, changed or deleted a compute, user, group (with its members and permissions), VM owner, flavor, setting, removed-VM record or SSH key, who mailed an OTP QR code, and every login, failed login, lockout and logout, with the address it came from and whether it worked (Result: Done, Failed, Unknown for older rows); a compute's VMs leaving the panel get a row each. On each compute it records changes to networks, storage pools, volumes (including clones and ISO uploads), host interfaces, secrets (never their values) and network filters.
 - The CPU usage in the new-VM dialog is the current load (over 1 s), read when the dialog opens; it was the average since the host booted.
 - 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.