tools/libxc: Tolerate specific zero-content records in migration v2 streams

tools/libxc: Tolerate specific zero-content records in migration v2 streams

The migration v2 save code was written to avoid sending data records with no
content, as such records serve no purpose but come with a performance hit.
The restore code sanity checks this expectation.

Under some circumstances (most notably, on AMD hardware with Debug Extensions,
and a PV guest kernel which is not using the feature), the save code would
generate a record with no content, which trips the sanity check in the restore
code.

As the stream is otherwise fine, tolerate these records and avoid failing the
migration.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>