dma_memory_rw() returns a MemTxResult type. Do not discard
it, return it to the caller.

Since dma_buf_rw() was previously returning the QEMUSGList
size not consumed, add an extra argument where this size
can be stored.

Update the 2 callers.

Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211223115554.3155328-14-philmd@redhat.com>