exec.h: fix coding style and change cpu_has_work to return bool

Before the next patch, fix coding style of the areas affected.

Change the type of the return value from cpu_has_work() and
qemu_cpu_has_work() to bool.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

cpu-exec.c

@@ -32,7 +32,7 @@ int tb_invalidated_flag;
 
 //#define CONFIG_DEBUG_EXEC
 
-int qemu_cpu_has_work(CPUState *env)
+bool qemu_cpu_has_work(CPUState *env)
 {
     return cpu_has_work(env);
 }