Documentation: Warn against qemu-img on active image

People have repeatedly expected that you can do things like snapshotting
an image with qemu-img while a qemu instance is running. Maybe we need
to consider locking the files while they are in use, but having a
warning in the qemu-img manpage is doable for 1.2 and can't hurt anyway.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-img.texi

@@ -4,6 +4,16 @@ usage: qemu-img command [command options]
 @c man end
 @end example
 
+@c man begin DESCRIPTION
+qemu-img allows you to create, convert and modify images offline. It can handle
+all image formats supported by QEMU.
+
+@b{Warning:} Never use qemu-img to modify images in use by a running virtual
+machine or any other process; this may destroy the image. Also, be aware that
+querying an image that is being modified by another process may encounter
+inconsistent state.
+@c man end
+
 @c man begin OPTIONS
 
 The following commands are supported: