virtio_random(8): MFC: avoid deadlock at shutdown time (regression fix)

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
Relnotes: yes
Approved by: re (gjb)

(cherry picked from commit adbf7727b3a2aad3c2faa6e543ee7fa7a6c9a3d5)
(cherry picked from commit 4a11315a2c3fc55333772f48aaef32ae1eb11ceb)