CA-91480: Cancel vm_import cleans up temporary objects

Xapi_vm_helpers.destroy would fail if a VM had a reference to a non-existant
blob, which would happen when the VM had been imported. This would prevent the
rest of the function from running, leaving ghost objects in the database after
an import was cancelled.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>
Merge pull request #1090 from mcclurmc/preq-911-clearwater

CA-91480: Cancel vm_import cleans up temporary objects