Add a third instance kind to the create wizard: a blank KVM VM with an
empty disk, the installation ISO mounted on ide2 and put first in the
boot order, so the guest boots straight into the installer over noVNC.
Picking a Windows guest presets OVMF, a SATA disk and an e1000 NIC,
which the installer sees without virtio drivers loaded.
Also teach the image downloader content=iso, so ISOs can be fetched
onto a node from the panel: by URL from the images page or from a saved
mirror (mirrors now accept kind=iso), and straight from the ISO step of
the wizard.
The bundled catalog stays qcow2-only on purpose — install ISOs have no
stable "latest" URLs, so hardcoded links would rot on every point release.
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