tests/avocado: set -machine none for userfwd and vnc tests

These are exercising core QEMU features and don't actually run code.
Not specifying a machine will fail when avocado chooses the native
arch binary to run. Be explicit.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20221027183637.2772968-19-alex.bennee@linaro.org>

tests/avocado/info_usernet.py

@@ -14,6 +14,9 @@ from qemu.utils import get_info_usernet_hostfwd_port
 
 
 class InfoUsernet(QemuSystemTest):
+    """
+    :avocado: tags=machine:none
+    """
 
     def test_hostfwd(self):
         self.require_netdev('user')

tests/avocado/vnc.py

@@ -53,6 +53,7 @@ def find_free_ports(count: int) -> List[int]:
 class Vnc(QemuSystemTest):
     """
     :avocado: tags=vnc,quick
+    :avocado: tags=machine:none
     """
     def test_no_vnc(self):
         self.vm.add_args('-nodefaults', '-S')