plugins: fix mem callback array size

plugins: fix mem callback array size

data was correctly copied, but size of array was not set
(g_array_sized_new only reserves memory, but does not set size).

As a result, callbacks were not called for code path relying on
plugin_register_vcpu_mem_cb().

Found when trying to trigger mem access callbacks for atomic
instructions.

Reviewed-by: Xingtao Yao <yaoxt.fnst@fujitsu.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240706191335.878142-2-pierrick.bouvier@linaro.org>
Message-Id: <20240718094523.1198645-6-alex.bennee@linaro.org>