virtio_random: Pipeline fetching the data

Queue an initial fetch of data during attach and after every read
rather than synchronously fetching data and polling for completion.

If data has not been returned from an previous fetch during read,
just return EAGAIN rather than blocking.

Co-authored-by: John Baldwin <jhb@FreeBSD.org>

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41656