accel/tcg: Avoid unnecessary call overhead from qemu_plugin_vcpu_mem_cb

accel/tcg: Avoid unnecessary call overhead from qemu_plugin_vcpu_mem_cb

If there are not any QEMU plugin memory callback functions, checking
before calling the qemu_plugin_vcpu_mem_cb function can reduce the
function call overhead.

Signed-off-by: Max Chou <max.chou@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-Id: <20240613175122.1299212-2-max.chou@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240620152220.2192768-13-alex.bennee@linaro.org>