Enable lguest drivers in Kconfig

Lguest drivers need to default to "Y" otherwise they're never selected
for new builds.  (We don't bother prompting, because they're less than
4k combined, and implied by selecting lguest support).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>