hw/char/goldfish: Use DMA memory API

hw/char/goldfish: Use DMA memory API

Rather than using address_space_rw(..., 0 or 1),
use the simpler DMA memory API which expand to
the same code. This allows removing a cast on
the 'buf' variable which is really const. Since
'buf' is only used in the CMD_READ_BUFFER case,
we can reduce its scope.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240723181850.46000-1-philmd@linaro.org>