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>
snapshot_disk: Fix block disk and raw disk external snapshot problem

When attach raw disk and use xml to do snapshot, update driver type
to qcow2 as it's the only support external snapshot format.

When domain disk is 'block' type, update disk type to 'file' and
source file to external snapshot file in snapshot xml.

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