Use a common tunable to choose between vt(4)/sc(4)

With this change and previous work from ray@ it will be possible to put
both in GENERIC, and have one enabled by default, but allow the other to
be selected via the loader.

(The previous implementation had separate kern.vt.disable and
hw.syscons.disable tunables, and would panic if both drivers were
compiled in and neither was explicitly disabled.)

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Add post-mortem debugging for "EPT Misconfiguration" VM-exit. This error
is hard to reproduce so try to collect all the breadcrumbs when it happens.

Reviewed by:	grehan