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: NEric Blake <eblake@redhat.com> Reviewed-by: NKevin Wolf <kwolf@redhat.com> Signed-off-by: NJeff Cody <jcody@redhat.com> Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
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