stream: pass new base image format to bdrv_change_backing_file

When an image is modified to point to the new backing file, the backing
file format is set to NULL, which means auto-probe.  This is wrong, in
fact it is a small security problem.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
