through the loop we look to see if signals are pending, and if so we return
early with an error. The caller (libvnode_xen) can use this to avoid really
long waits, when the server has said to stop what its doing. For example, a
vnode setup is waiting for an image lock, but the server comes along ands
to stop setting up. Previously, we would wait for the lock, now we return
early. This is to help with cancelation where it is nice if the server can
stop a CreateSliver() in its tracks, when it is safe to do so.