Turn off Sphinx nitpicking as a temporary (?) measure so
sphinx builds complete even with warnings about missing
references.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20190521122519.12573-11-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>