snapshot_create_as: Fix external snapshot file clean problem

By default, memory external snapshot without diskspec option is
external system checkpoint snapshot (both external memory and disk
snapshot).

Update filter 'live_memspec' with specific diskspec, which will
get the external disk snapshot file checked and cleaned.

Update filter 'no_metadata_with_memspec' with 'snapshot=no' in
diskspec, which only do memory external snapshot.

Signed-off-by: Wayne Sun <gsun@redhat.com>