Split Qemu parameters

So we can re-use params for incremental install

artiq-fast/windows/win.nix

@@ -113,22 +113,34 @@ let
       virt-make-fs --partition --type=fat pkgs/ $out
     '';
 
+  mkQemuFlags = extraFlags: [
+    "-enable-kvm"
+    "-cpu"
+    "host"
+    "-smp"
+    "$NIX_BUILD_CORES"
+    "-m"
+    "${qemuMem}"
+    "-bios"
+    "${pkgs.OVMF.fd}/FV/OVMF.fd"
+    "-vga"
+    "virtio"
+    "-device"
+    "piix3-usb-uhci" # USB root hub
+    # "CD" drive with windows features-on-demand
+    # "-cdrom" "${fodIso}"
+    # Set the base clock inside the VM
+    "-rtc base=${baseRtc}"
+  ] ++ lib.optional (!impureMode) "-nographic" ++ extraFlags;
+
   installScript = pkgs.writeScript "windows-install-script" (
     let
-      qemuParams = [
-        "-enable-kvm"
-        "-cpu"
-        "host"
-        "-smp"
-        "$NIX_BUILD_CORES"
-        "-m"
-        "${qemuMem}"
-        "-bios"
-        "${pkgs.OVMF.fd}/FV/OVMF.fd"
-        "-vga"
-        "virtio"
+      qemuParams = mkQemuFlags [
+        # "CD" drive with bootstrap pkgs
+        "-drive"
+        "id=virtio-win,file=${bootstrapPkgs},if=none,format=raw,readonly=on"
         "-device"
-        "piix3-usb-uhci" # USB root hub
+        "usb-storage,drive=virtio-win"
         # USB boot
         "-drive"
         "id=win-install,file=usbimage.img,if=none,format=raw,readonly=on"
@@ -137,22 +149,17 @@ let
         # Output image
         "-drive"
         "file=c.img,index=0,media=disk,cache=unsafe"
-        # "CD" drive with bootstrap pkgs
-        "-drive"
-        "id=virtio-win,file=${bootstrapPkgs},if=none,format=raw,readonly=on"
-        "-device"
-        "usb-storage,drive=virtio-win"
-        # "CD" drive with windows features-on-demand
-        # "-cdrom" "${fodIso}"
-        # Set the base clock inside the VM
-        "-rtc=$(date -Iseconds -d @${baseRtc}"
-      ] ++ lib.optional (!impureMode) "-nographic";
+      ];
     in
       ''
         #!${pkgs.runtimeShell}
         set -euxo pipefail
         export PATH=${lib.makeBinPath [ p7zip qemu libguestfs ]}:$PATH
 
+        if test -z "''${NIX_BUILD_CORES+x}"; then
+          export NIX_BUILD_CORES=$(nproc)
+        fi
+
         # Create a bootable "USB" image
         # Booting in USB mode circumvents the "press any key to boot from cdrom" prompt
         #