Split the QEMU configs by host system and architecture

Replace qemu/<guest>-<arch>.ini with qemu/<guest>-<os>-<arch>.ini, where os is
linux or darwin, the name platform.system() reports. The accelerators move out
of the shared <guest>.ini into these files (KVM on Linux, HVF on macOS, TCG as
the fallback). The launchers pick the file from the host system and
architecture, and reject amd64 guests on macOS hosts, so no darwin-amd64 files
exist.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

qemu/ubuntu2604-linux-arm64.ini

@@ -0,0 +1,24 @@
+# Linux host, arm64 part of the Ubuntu 26.04 QEMU configuration; read after ubuntu2604.ini.
+
+# Accelerators are tried in order: KVM on Linux, else emulation.
+[accel]
+  accel = "kvm"
+
+[accel]
+  accel = "tcg"
+
+[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"