cpu: Turn cpu_has_work() into a CPUClass hook

Default to false.

Tidy variable naming and inline cast uses while at it.

Tested-by: Jia Liu <proljc@gmail.com> (or32)
Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/cpus.c b/cpus.c
index b6421fd..eda6d02 100644
--- a/cpus.c
+++ b/cpus.c
@@ -76,7 +76,7 @@
     if (cpu_is_stopped(cpu)) {
         return true;
     }
-    if (!cpu->halted || qemu_cpu_has_work(cpu) ||
+    if (!cpu->halted || cpu_has_work(cpu) ||
         kvm_halt_in_kernel()) {
         return false;
     }