vt(4): Fix keyboard allocation when kbdmux(4) isn't used

The problem was that only the kbdmux keyboard index was saved in
vd->vd_keyboard. This index is -1 when kbdmux isn't used. In this
case, the keyboard was correctly allocated, but the returned index was
discarded.

PR:		194718
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273973 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f