Remove passt networking mode support

[kvinwang]

Summary

• Completely remove passt networking mode from VMM
• Remove passt-related config fields, CLI options, qemu config generation, and process lifecycle management
• Passt processes were leaking in supervisor because stop_vm and remove_vm didn't clean them up; removing the mode entirely eliminates this class of bugs

Changed files

• vmm/src/config.rs — Remove NetworkingMode::Passt, passt struct fields, is_passt()
• vmm/src/app.rs — Remove passt stop/remove logic from start_vm and remove_vm
• vmm/src/app/qemu.rs — Remove config_passt(), Passt match arm, passt workdir methods
• vmm/src/main_service.rs — Remove passt from proto conversion
• vmm/src/vmm-cli.py — Remove 'passt' from --net choices
• vmm/src/console_v1.html — Remove Passt option from UI dropdown
• vmm/vmm.toml — Remove passt config section
• vmm/rpc/proto/vmm_rpc.proto — Update comment

Test plan

• cargo fmt and cargo clippy pass
• Verify existing VMs with user and bridge networking modes still work