block: add BlockLimits.max_iov field

The maximum number of struct iovec elements depends on the
BlockDriverState.  The raw-posix and iSCSI protocols have a maximum of
IOV_MAX but others could have different values.

Cc: Peter Lieven <pl@kamp.de>
Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 9a1c466..256609d 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -330,6 +330,9 @@
 
     /* memory alignment for bounce buffer */
     size_t opt_mem_alignment;
+
+    /* maximum number of iovec elements */
+    int max_iov;
 } BlockLimits;
 
 typedef struct BdrvOpBlocker BdrvOpBlocker;