plugins: Move function pointer in qemu_plugin_dyn_cb

plugins: Move function pointer in qemu_plugin_dyn_cb

The out-of-line function pointer is mutually exclusive
with inline expansion, so move it into the union.
Wrap the pointer in a structure named 'regular' to match
PLUGIN_CB_REGULAR.

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