vt: Add devctl message for bells

Generate VT events when the bell beeps. When coupled with disabling the
bell,this allows custom bells to be rung when we'd otherwise beep.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D32656

(cherry picked from commit 4ac3d08a9693c27c1bd2ddd67b2808ac9e18f4c5)