Add a virtio-input device emulation.

This will be used to inject keyboard/mouse input events into a guest.
The command line syntax is:

-s <slot>,virtio-input,/dev/input/eventX

Reviewed by: jhb (bhyve), grehan
Obtained from: Corvin Köhne <C.Koehne@beckhoff.com>
MFC after: 3 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D30020

(cherry picked from commit 054accac71e0944ab588c3ab052bea6947df7885)