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/windows11-linux-arm64.ini

@@ -0,0 +1,34 @@
+# Linux host, arm64 part of the Windows 11 QEMU configuration; read after windows11.ini.
+
+# Accelerators are tried in order: KVM on Linux, else emulation.
+[accel]
+  accel = "kvm"
+
+[accel]
+  accel = "tcg"
+
+[machine]
+  type = "virt"
+
+# The "virt" machine has no IDE. NVMe and USB mass storage (used by the
+# installation media of --install-from-iso) both have inbox Windows drivers.
+[device "system_disk-dev"]
+  driver = "nvme"
+  drive = "system_disk"
+  serial = "win11disk"
+  bootindex = "1"
+
+# Windows has no inbox driver for virtio-net; the guest tools installed by
+# Configure-Windows.ps1 provide it.
+[device "nic0"]
+  driver = "virtio-net-pci"
+  netdev = "net0"
+
+# Unlike q35, "virt" has no default GPU or keyboard. ramfb is used rather than
+# virtio-gpu because the firmware's virtio-gpu output freezes at the Windows
+# boot manager; Windows drives either one through its basic display driver.
+[device "gpu0"]
+  driver = "ramfb"
+
+[device "kbd0"]
+  driver = "usb-kbd"