tests/avocado: Do not run tests that require libslirp if it is not available

Some avocado tests blindly assume that QEMU has been compiled with libslirp
enabled and fail badly if it is missing. Add a proper check to cancel the
tests in this case.

Message-Id: <20220824151122.704946-6-thuth@redhat.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/avocado/replay_linux.py

@@ -55,6 +55,7 @@ class ReplayLinux(LinuxTest):
             '%s,drive=disk%s-rr%s' % (device, id, bus_string))
 
     def launch_and_wait(self, record, args, shift):
+        self.require_netdev('user')
         vm = self.get_vm()
         vm.add_args('-smp', '1')
         vm.add_args('-m', '1024')