cpus-common: move CPU list management to common code

Add a mutex for the CPU list to system emulation, as it will be used to
manage safe work.  Abstract manipulation of the CPU list in new functions
cpu_list_add and cpu_list_remove.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/exec/cpu-common.h

@@ -23,6 +23,11 @@ typedef struct CPUListState {
     FILE *file;
 } CPUListState;
 
+/* The CPU list lock nests outside tb_lock/tb_unlock.  */
+void qemu_init_cpu_list(void);
+void cpu_list_lock(void);
+void cpu_list_unlock(void);
+
 #if !defined(CONFIG_USER_ONLY)
 
 enum device_endian {