block/qcow2: Generalize preallocate()
This patch adds two new parameters to the preallocate() function so we will be able to use it not just for preallocating a new image but also for preallocated image growth. The offset parameter allows the caller to specify a virtual offset from which to start preallocating. For newly created images this is always 0, but for preallocating growth this will be the old image length. The new_length parameter specifies the supposed new length of the image (basically the "end offset" for preallocation). During image truncation, bdrv_getlength() will return the old image length so we cannot rely on its return value then. Signed-off-by: NMax Reitz <mreitz@redhat.com> Reviewed-by: NEric Blake <eblake@redhat.com> Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com> Message-id: 20170613202107.10125-10-mreitz@redhat.com Signed-off-by: NMax Reitz <mreitz@redhat.com>
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