block: fix external snapshot abort permission error

In external_snapshot_abort(), we try to undo what was done in
external_snapshot_prepare() calling bdrv_replace_node() to swap the
nodes back.  However, we receive a permissions error as writers are
blocked on the old node, which is now the new node backing file.

An easy fix (initially suggested by Kevin Wolf) is to call
bdrv_set_backing_hd() on the new node, to set the backing node to NULL.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>