dm ima: add version info to dm related events in ima log

The DM events present in the ima log contain various attributes in the
key=value format.  The attributes' names/values may change in future,
and new attributes may also get added.  The attestation server needs
some versioning to determine which attributes are supported and are
expected in the ima log.

Add version information to the DM events present in the ima log to
help attestation servers to correctly process the attributes across
different versions.

Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>