virtio: add loader tunables to sysctl

virtio_pci uses two loader tunables that should be more visible.
This patch adds these loader tunables to sysctl and describes them
in the virtio(4) man page.

Reviewed by: imp (erlier version), tuexen
Differential Revision: https://reviews.freebsd.org/D55533
Event: Wiesbaden Hackathon 2026

(cherry picked from commit c70755bc0d8f703dbaa1520c15e8213a95847dd5)