CA-97307: recover from the TVM disk being deleted

Currently, when we need to expose a VM disk image, we check to see if the TVM template has been installed. If it hasn't been installed, then we go ahead and import the new image. If however, the template exists, but no longer has any VBDs (perhaps the user has deleted the VDI), then we do not re-import the TVM.

This patch set fixes that problem, and checks the template has VBDs - if it doesn't - then it goes ahead and installs a new template (cleaning up the stale one).

Signed-off-by: Rob Dobson <rob@rdobson.co.uk>