SCTX-1515: Best effort VDI ops when DB rows disappear.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #1647 from ravippandey/Snowball/HFX-1042

[Snowball/HFX-1042] SCTX-1515: Best effort VDI ops when DB rows disappear.