MFC: r305507

MFC: r305507

Disable vt(4) by default on sparc64 as creator_vt(4) and vt_ofwfb(4)
have the serious problem of not actually attaching the hardware they
are driving at the bus level. This causes creator(4) and machfb(4)
to attach and drive the very same hardware in parallel when both
syscons(4) and vt(4) as well as their associated hardware drivers
are built into a kernel, i. e. GENERIC, at the same time.
Also, syscons(4) and its drivers still are way superior to vt(4) and
its equivalents; unlike the syscons(4) counterparts the vt(4) drivers
don't provide hardware acceleration resulting in considerably slower
screen drawing, creator_vt(4) doesn't provide a /dev/fb node as
required by the Xorg sunffb(4) etc. In theory, vt_ofwfb(4) should be
able to handle more devices than machfb(4). However, testing shows
that it hardly works with any hardware machfb(4) isn't also able to
drive, making vt(4) and vt_ofwfb(4) not favorable for the time being
from that perspective either.