dma-helpers: add dma_buf_read and dma_buf_write

These helpers do a full transfer from an in-memory buffer to target
memory, with support for scatter/gather lists.  It will be used to
store the reply of an emulated command into a QEMUSGList provided by
the adapter.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/dma.h b/dma.h
index d50019b..346ac4f 100644
--- a/dma.h
+++ b/dma.h
@@ -58,4 +58,7 @@
 BlockDriverAIOCB *dma_bdrv_write(BlockDriverState *bs,
                                  QEMUSGList *sg, uint64_t sector,
                                  BlockDriverCompletionFunc *cb, void *opaque);
+uint64_t dma_buf_read(uint8_t *ptr, int32_t len, QEMUSGList *sg);
+uint64_t dma_buf_write(uint8_t *ptr, int32_t len, QEMUSGList *sg);
+
 #endif