coroutine-ucontext: use helper for allocating stack memory

typedef struct {

    Coroutine base;

    void *stack;

    size_t stack_size;

    sigjmp_buf env;

#ifdef CONFIG_VALGRIND_H

Coroutine *qemu_coroutine_new(void)

{

    const size_t stack_size = COROUTINE_STACK_SIZE;

    CoroutineUContext *co;

    ucontext_t old_uc, uc;

    sigjmp_buf old_env;

    }

    co = g_malloc0(sizeof(*co));

    co->stack = g_malloc(stack_size);

    co->stack_size = COROUTINE_STACK_SIZE;

    co->stack = qemu_alloc_stack(&co->stack_size);

    co->base.entry_arg = &old_env; /* stash away our jmp_buf */

    uc.uc_link = &old_uc;

    uc.uc_stack.ss_sp = co->stack;

    uc.uc_stack.ss_size = stack_size;

    uc.uc_stack.ss_size = co->stack_size;

    uc.uc_stack.ss_flags = 0;

#ifdef CONFIG_VALGRIND_H

    co->valgrind_stack_id =

        VALGRIND_STACK_REGISTER(co->stack, co->stack + stack_size);

        VALGRIND_STACK_REGISTER(co->stack, co->stack + co->stack_size);

#endif

    arg.p = co;

    valgrind_stack_deregister(co);

#endif

    g_free(co->stack);

    qemu_free_stack(co->stack, co->stack_size);

    g_free(co);

}