In get_physical_address(), use address_space_ldl() and
address_space_stl() instead of ldl_phys() and stl_phys().
This allows us to check whether the memory access failed.
For the moment, we simply return -1 in this case;
add a TODO comment that we should ideally generate the
appropriate kind of fault.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20181210165636.28366-3-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>