target/alpha: Switch to do_transaction_failed() hook

Switch the alpha target from the old unassigned_access hook
to the new do_transaction_failed hook. This allows us to
resolve a ??? in the old hook implementation.

The only part of the alpha target that does physical
memory accesses is reading the page table -- add a
TODO comment there to the effect that we should handle
bus faults on page table walks. (Since the palcode
doesn't actually do anything useful on a bus fault anyway
it's a bit moot for now.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1502196172-13818-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>