vt(4): Return errors from vt_{,de}allocate()

This is useful to the DRM drivers to let them know if a device is
effectively used by the console.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38089

(cherry picked from commit 29ab19455484be96150fa5e87bd38aefb85c888a)