test-coroutine: prepare for the next patch

The next patch moves the coroutine argument from first-enter to
creation time.  In this case, coroutine has not been initialized
yet when the coroutine is created, so change to a pointer.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>