default-configs: Replace $(and ...) with $(call land, ...)

Using $(and ...) is dangerous here: It only works as long as the first
argument is set to 'y' or completely unset. It does not work if the
first argument is set to 'n' for example. Let's use the "land" make
function instead which has been written explicitely for this purpose.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1505759538-15365-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>