trace: improve error reporting when parsing simpletrace header

When loading a simpletrace binary file we just report
"Not a valid trace file!" which is not very helpful. Report
exactly which field we found to be invalid.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170125161417.31949-9-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
