tests/vm/ubuntu.aarch64: Correct comment about TCG specific delay

Wether we use a software MMU or not to set the SSH timeout
isn't really relevant. What we want to know is if we use
a hardware or software accelerator (TCG).
Replace the 'softmmu' mention by 'TCG'.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20231002145104.52193-2-philmd@linaro.org>

tests/vm/ubuntu.aarch64

@@ -25,7 +25,7 @@ DEFAULT_CONFIG = {
                      "apt-get install -y libfdt-dev pkg-config language-pack-en ninja-build",
     # We increase beyond the default time since during boot
     # it can take some time (many seconds) to log into the VM
-    # especially using softmmu.
+    # especially using TCG.
     'ssh_timeout'  : 60,
 }