CA-111132: Make xcp-rrdd-iostat handle empty VBDs

Previously, if a VM has at least one empty VBD, none of the VDIs for this VM would be stored in the VDI-to-VM map. Now, empty VBDs are simply ignored.
Merge pull request #1 from jjd27/master

CA-111132: Make xcp-rrdd-iostat handle empty VBDs