coroutine-sleep: disallow NULL QemuCoSleepState** argument

Simplify the code by removing conditionals.  qemu_co_sleep_ns
can simply point the argument to an on-stack temporary.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20210517100548.28806-3-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>