kvm: fix exception in volume stats after storage migration

[ustcweizhou]

Description

on kvm, the 'path' of volume is the file name on primary storage. we should use 'path' instead of 'uuid' in volume statistics.

Fixes: #3878

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)

Screenshots (if appropriate):

How Has This Been Tested?

1. storage migration
2. volume statistics every 10 minutes (default)