tests/avocado/virtio-gpu: Cancel test if drm rendering is not available

The test_vhost_user_vga_virgl test currently fails on some CI
machines with:

 qemu-system-x86_64: egl: no drm render node available
 qemu-system-x86_64: egl: render node init failed

The other test in this file already checks whether there is
an error while starting QEMU - we should do the same for the
test_vhost_user_vga_virgl test, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230530180330.48722-1-thuth@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tests/avocado/virtio-gpu.py

@@ -143,7 +143,11 @@ def test_vhost_user_vga_virgl(self):
             "-append",
             self.KERNEL_COMMAND_LINE,
         )
-        self.vm.launch()
+        try:
+            self.vm.launch()
+        except:
+            # TODO: probably fails because we are missing the VirGL features
+            self.cancel("VirGL not enabled?")
         self.wait_for_console_pattern("as init process")
         exec_command_and_wait_for_pattern(
             self, "/usr/sbin/modprobe virtio_gpu", ""