libxc/migration: Rationalise the 'checkpointed' field to 'stream_type'

libxc/migration: Rationalise the 'checkpointed' field to 'stream_type'

Originally, 'checkpointed' was a boolean signalling the difference between a
plain and a Remus stream.  COLO was added later, but several bits of code
retained boolean-style logic.  While correct, it is confusing to follow.

Additionally, XC_MIG_STREAM_NONE means "no checkpoints" but reads as "no
stream".

Consolidate all the logic on the term 'stream_type', and rename STREAM_NONE
to STREAM_PLAIN.  Re-position the stream_type variable so it isn't
duplicated in both the save and restore unions.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>