clean root dir of all x86-ness

Remove all references to x86 from the root dir (except from in
configure).  Also remove references from the root dir README
by moving that documentation to the x86/README, and touch up
the READMEs at the same time.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
[Drop --test-dir, adjust includes. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>