virtio_random(8): avoid deadlock at shutdown time

FreeBSD 13+ running as virtual guest may load virtio_random(8) driver
by means of devd(8) unless the driver is blacklisted or disabled
via device.hints(5). Currently, the driver may prevent
the system from rebooting or shutting down correctly.

This change deactivates virtio_random at very late stage
during system shutdown sequence to avoid deadlock
that results in kernel hang.

PR: 253175
Tested by: tom
MFC after: 3 days