dataplane: Mark host notifiers' client type as "external"

They will be excluded by type in the nested event loops in block layer,
so that unwanted events won't be processed there.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
