fix 100% cpu utilization when cpu is stopped

> Hello,
>
> d6f4ade (disentangle tcg and deadline calculation, 2010-03-10)
> introduces following regression(s):
>
> 100% cpu utilization when QEMU is invoked like:
> qemu -S -s ...
>
> ditto when gdb takes control over the session via gdb-stub
> (i.e. the breakpoint is hit or C-c is pressed inside gdb to
>   interrupt the attached qemu instance)

The bug is that env->stopped is not really as comprehensive as it seems to
be (and cpu_has_work thinks); it is only valid with iothread basically,
and even then it is cleared by reset and it is not set when starting
qemu with -S.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: malc <av1474@comtv.ru>
Signed-off-by: malc <av1474@comtv.ru>
