virtio_random(4): Fix random(4) integration

random(4) masks unregistered entropy sources. Prior to this revision,
virtio_random(4) did not correctly register a random_source and did not
function as a source of entropy.

Random source registration for loadable pure sources requires registering a
poll callback, which is invoked periodically by random(4)'s harvestq
kthread. The periodic poll makes virtio_random(4)'s periodic entropy
collection redundant, so this revision removes the callout.

The current random source API is somewhat limiting, so simply fail to attach
any virtio_random devices if one is already registered as a source. This
scenario is expected to be uncommon.

While here, handle the possibility of short reads from the hypervisor random
device gracefully / correctly. It is not clear why a hypervisor would
return a short read or if it is allowed by spec, but we may as well handle
it.

Reviewed by: bryanv (earlier version), markm
Security: yes (note: many other "pure" random sources remain broken)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20419