The -n flag requests QEMU to reuse the image found in new-image-file,
instead of recreating it from scratch.
The -f flag requests QEMU to copy the whole disk, so that the result
does not need a backing file. Note that this flag *must* currently be
passed since the other sync modes ('none' and 'top') have not been
implemented yet. Requiring it ensures that "drive_backup" behaves like
"drive_mirror".
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>