exec: drop cpu_can_do_io, just read cpu->can_do_io

After commit 626cf8f (icount: set can_do_io outside TB execution,
2014-12-08), can_do_io is set to 1 if not executing code.  It is
no longer necessary to make this assumption in cpu_can_do_io.

It is also possible to remove the use_icount test, simply by
never setting cpu->can_do_io to 0 unless use_icount is true.

With these changes cpu_can_do_io boils down to a read of
cpu->can_do_io.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/cpus.c b/cpus.c
index a822ce3..c1e74d9 100644
--- a/cpus.c
+++ b/cpus.c
@@ -145,7 +145,7 @@
 
     icount = timers_state.qemu_icount;
     if (cpu) {
-        if (!cpu_can_do_io(cpu)) {
+        if (!cpu->can_do_io) {
             fprintf(stderr, "Bad icount read\n");
             exit(1);
         }