dma-helpers: make QEMUSGList target independent

scsi-disk will manage scatter/gather list, but it does not create
single entries so it remains target-independent.  Make QEMUSGList
available to it.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

dma.h

@@ -17,6 +17,13 @@
 
 typedef struct ScatterGatherEntry ScatterGatherEntry;
 
+struct QEMUSGList {
+    ScatterGatherEntry *sg;
+    int nsg;
+    int nalloc;
+    size_t size;
+};
+
 #if defined(TARGET_PHYS_ADDR_BITS)
 typedef target_phys_addr_t dma_addr_t;
 
@@ -32,13 +39,6 @@ struct ScatterGatherEntry {
     dma_addr_t len;
 };
 
-struct QEMUSGList {
-    ScatterGatherEntry *sg;
-    int nsg;
-    int nalloc;
-    dma_addr_t size;
-};
-
 void qemu_sglist_init(QEMUSGList *qsg, int alloc_hint);
 void qemu_sglist_add(QEMUSGList *qsg, dma_addr_t base, dma_addr_t len);
 void qemu_sglist_destroy(QEMUSGList *qsg);