temporary fix for on_vcpu

temporary fix for on_vcpu

Recent changes made on_vcpu hit the abort() path, even with the IO thread
disabled. This is because cpu_single_env is no longer set when we call this
function. Although the correct fix is a little bit more complicated that that,
the recent thread in which I proposed qemu_queue_work (which fixes that, btw),
is likely to go on a quite different direction.

So for the benefit of those using guest debugging, I'm proposing this simple
fix in the interim.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>