Unable to remove SPICE channels in GUI – breaks VNC-only VM setups (esp. for Windows guests)

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Problem

When creating a new VM in Cockpit with the intention of using VNC only (no SPICE), Cockpit still automatically includes SPICE-related components like <channel type='spicevmc'>. With recent libvirt versions, this results in a boot failure if no <graphics type='spice'> block is defined.

Even when adding a <graphics type='vnc'> block manually, VMs fail to start unless SPICE-related channels are manually removed via virsh edit.

Expected Behavior

Cockpit should allow users to:

• Choose VNC-only during VM creation
• Omit SPICE-related <channel> devices when SPICE is not used
• Avoid inserting conflicting graphics definitions (e.g., both SPICE and VNC)
• Successfully create and boot VMs like Windows 11 using VNC + TPM without XML editing

Why it matters

• Windows 11 requires TPM, UEFI, and VirtIO — Cockpit supports these nicely, but the graphics issue blocks install.
• This forces users into complex XML edits or CLI workflows, defeating the purpose of a GUI.
• libvirt errors are cryptic for new users: e.g., chardev 'spicevmc' not supported without spice graphics.

Environment

• Debian 12 (Bookworm)
• Cockpit 329 from backports
• libvirt 9.0.0
• QEMU 7.2
• OVMF and swtpm installed

Request

Please provide a checkbox or dropdown to choose graphics type (SPICE or VNC), and omit unnecessary channels/devices accordingly.

Thanks for all the hard work — Cockpit is awesome otherwise!