tests/functional: Restrict nested Aarch64 Xen test to TCG

Currently QEMU only support accelerating EL0 and EL1, so features
requiring EL2 (like virtualization) or EL3 must be emulated with TCG.

On macOS this test fails:

  qemu-system-aarch64: mach-virt: HVF does not support providing Virtualization extensions to the guest CPU

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250623121845.7214-24-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

tests/functional/test_aarch64_xen.py

@@ -33,6 +33,7 @@ class BootXen(LinuxKernelTest):
         """
         Launch Xen with a dom0 guest kernel
         """
+        self.require_accelerator("tcg") # virtualization=on
         self.set_machine('virt')
         self.cpu = "cortex-a57"
         self.kernel_path = self.ASSET_KERNEL.fetch()