block: Image file option amendment

This patch adds the "amend" option to qemu-img which allows changing
image options on existing image files. It also adds the generic bdrv
implementation which is basically just a wrapper for the image format
specific function.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/qemu-img.texi b/qemu-img.texi
index 43ee4eb..768054e 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -350,6 +350,11 @@
 After using this command to grow a disk image, you must use file system and
 partitioning tools inside the VM to actually begin using the new space on the
 device.
+
+@item amend [-f @var{fmt}] -o @var{options} @var{filename}
+
+Amends the image format specific @var{options} for the image file
+@var{filename}. Not all file formats support this operation.
 @end table
 @c man end