block: Move BlockRequest type to io.c

I was thrown by the fact that the public type BlockRequest had
an anonymous union, but no obvious discriminator.  Turns out
that the only client of the second branch of the union was code
internal to io.c, now that commit 91c6e4b killed public
multiwrite, so move it into io.c and improve the comments.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1463699150-19445-1-git-send-email-eblake@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

block/io.c

@@ -1902,6 +1902,27 @@ void bdrv_aio_cancel_async(BlockAIOCB *acb)
 /**************************************************************/
 /* async block device emulation */
 
+typedef struct BlockRequest {
+    union {
+        /* Used during read, write, trim */
+        struct {
+            int64_t sector;
+            int nb_sectors;
+            int flags;
+            QEMUIOVector *qiov;
+        };
+        /* Used during ioctl */
+        struct {
+            int req;
+            void *buf;
+        };
+    };
+    BlockCompletionFunc *cb;
+    void *opaque;
+
+    int error;
+} BlockRequest;
+
 typedef struct BlockAIOCBCoroutine {
     BlockAIOCB common;
     BlockRequest req;