Load the Debian 13 VM hardware from QEMU config files

Move the Debian launcher's QEMU arguments into qemu/debian13.ini, which
the launcher renders with the per-run file paths and passes to QEMU with
-readconfig, followed by qemu/debian13-<arch>.ini for the machine type
and arm64-only devices. Memory, CPU count, SSH port and accelerator are
now set in the config files, replacing VM_MEMORY, VM_CPUS and
VM_SSH_PORT; VM_QEMU_CONFIG selects a different shared config file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YKxXLyiL7XJ3a9e8HmR2my

qemu/debian13-arm64.ini

@@ -0,0 +1,17 @@
+# arm64 part of the Debian 13 QEMU configuration; read after debian13.ini.
+
+[machine]
+  type = "virt"
+
+# Unlike q35, the "virt" machine has no default GPU or USB input devices.
+[device "gpu0"]
+  driver = "virtio-gpu-pci"
+
+[device "usb0"]
+  driver = "qemu-xhci"
+
+[device "kbd0"]
+  driver = "usb-kbd"
+
+[device "tablet0"]
+  driver = "usb-tablet"