Use pow2floor() to round down to the nearest power of 2,
rather than an inline calculation.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1437741192-20955-5-git-send-email-peter.maydell@linaro.org