Restrict cpu_exec_interrupt() and its callees to sysemu.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210911165434.531552-22-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>