configure: improve error for ucontext coroutine backend

configure: improve error for ucontext coroutine backend

Instead of using feature_not_found(), which is not a good match because
there is no "remedy" to fix the lack of makecontext(), just print a
custom error.

This happens to remove the last use of feature_not_found(), so remove
the definition and the documentation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>