system: Move 'exec/confidential-guest-support.h' to system/

system: Move 'exec/confidential-guest-support.h' to system/

"exec/confidential-guest-support.h" is specific to system
emulation, so move it under the system/ namespace.
Mechanical change doing:

  $ sed -i \
    -e 's,exec/confidential-guest-support.h,sysemu/confidential-guest-support.h,' \
        $(git grep -l exec/confidential-guest-support.h)

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Message-Id: <20241218155913.72288-2-philmd@linaro.org>