qemu-coroutine: reserve pool variables only if pool is enabled

Signed-off-by: Peter Lieven <pl@kamp.de>
qemu-coroutine: fix coroutine_pool_cleanup_local

Signed-off-by: Peter Lieven <pl@kamp.de>
coroutine-ucontext: safe memset on new coroutine

Signed-off-by: Peter Lieven <pl@kamp.de>