Change log from ERROR to WARNING when instance absent

Periodic task will update the host stat so that it will
collect all active instances and get their disk size
during the operation period, the instance might be resized
by nova operations, it will lead to disk file not exist
it's not an error from current design point of view, so
change from ERROR to WARNING to avoid confusion

Change-Id: I3b6f1416c248d9d4d2caaf2cc30a7c944685661c
Closes-Bug: #1243849
Merge "Change log from ERROR to WARNING when instance absent"