qemu-img: Specify backing file for commit

Introduce a new parameter for qemu-img commit which may be used to
explicitly specify the backing file into which an image should be
committed if the backing chain has more than a single layer.

[Applied Eric Blake's qemu-img.texi documentation rewording
--Stefan]

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1414159063-25977-12-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index fde33d1..04c207d 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -22,9 +22,9 @@
 ETEXI
 
 DEF("commit", img_commit,
-    "commit [-q] [-f fmt] [-t cache] [-d] [-p] filename")
+    "commit [-q] [-f fmt] [-t cache] [-b base] [-d] [-p] filename")
 STEXI
-@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-d] [-p] @var{filename}
+@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename}
 ETEXI
 
 DEF("compare", img_compare,