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block: add QAPI command to allow live backing file change
This allows a user to make a live change to the backing file recorded in
an open image.
The image file to modify can be specified 2 ways:
1) image filename
2) image node-name
Note: this does not cause the backing file itself to be reopened; it
merely changes the backing filename in the image file structure, and
in internal BDS structures.
It is the responsibility of the user to pass a filename string that
can be resolved when the image chain is reopened, and the filename
string is not validated.
A good analogy for this command is that it is a live version of
'qemu-img rebase -u', with respect to changing the backing file string.
[Jeff is offline so I respun this patch in his absence. Dropped image
filename since using node-name is preferred and this is a new command.
No need to introduce the limitations of finding images by filename.
--Stefan]
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>