CA-74544: Associate a username with the sessions xapi passes to SM backends.

This will appear in the audit log. Formatting is consistent with the
"__dom0__vmpr" username associated with the session passed to the VMPR
plugin.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1005 from johnelse/ca-74544

CA-74544: Associate a username with the sessions xapi passes to SM backends.