Don't log every readonly API call.

This causes the periodic SR.scans to produce a lot less log spam - these
lines make up about 20% of xensource.log at the moment if there is an SR
being autoscanned.

Signed-off-by: John Else <john.else@citrix.com>
Remove debugging.

This line gets logged three times for each SR.scan (to both
/var/log/xensource.log and /var/log/messages), and the information is
already available in the SM.features field in the DB.

Signed-off-by: John Else <john.else@citrix.com>
Remove unused module imports

Signed-off-by: John Else <john.else@citrix.com>