add qemu-img convert -n option (skip target volume creation)

Add a -n option to skip volume creation on qemu-img convert.
This is useful for targets such as rbd / ceph, where the
target volume may already exist; we cannot always rely on
qemu-img convert to create the image, as dependent on the
output format, there may be parameters which are not possible
to specify through the qemu-img convert command line.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/qemu-img.texi b/qemu-img.texi
index 69f1bda..ad45a6d 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -96,6 +96,14 @@
 Strict mode - fail on on different image size or sector allocation
 @end table
 
+Parameters to convert subcommand:
+
+@table @option
+
+@item -n
+Skip the creation of the target volume
+@end table
+
 Command description:
 
 @table @option
@@ -171,7 +179,7 @@
 
 @end table
 
-@item convert [-c] [-p] [-f @var{fmt}] [-t @var{cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
+@item convert [-c] [-p] [-n] [-f @var{fmt}] [-t @var{cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
 
 Convert the disk image @var{filename} or a snapshot @var{snapshot_name} to disk image @var{output_filename}
 using format @var{output_fmt}. It can be optionally compressed (@code{-c}
@@ -190,6 +198,11 @@
 @var{backing_file} should have the same content as the input's base image,
 however the path, image format, etc may differ.
 
+If the @code{-n} option is specified, the target volume creation will be
+skipped. This is useful for formats such as @code{rbd} if the target
+volume has already been created with site specific options that cannot
+be supplied through qemu-img.
+
 @item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}
 
 Give information about the disk image @var{filename}. Use it in