backends/tpm: Avoid using g_alloca()

backends/tpm: Avoid using g_alloca()

tpm_emulator_ctrlcmd() is not in hot path.
Use the heap instead of the stack, removing
the g_alloca() call.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20250605193540.59874-3-philmd@linaro.org>