virtual_oss(8): Use kldload(2) instead of system(3)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: imp, markj, emaste
Differential Revision: https://reviews.freebsd.org/D53617
(cherry picked from commit 7bedc4634d893174b9cef06c7c13e5f1cb24808b)