Fix regression with live migration on shared storage

The commit c1ccc1a316 introduced
a regression when NUMA live migration was done on shared storage

The live migration support for the power mgmt feature means we need to
call driver.cleanup() for all NUMA instances to potentially offline
pcpus that are not used any more after the instance is migrated away.
However this change exposed an issue with the disk cleanup logic. Nova
should never delete the instance directory if that directory is on
shared storage (e.g. the nova instances path is backed by NFS).

This patch will fix that behavior so live migration will function

Closes-Bug: #2080436
Change-Id: Ia2bbb5b4ac728563a8aabd857ed0503449991df1

releasenotes/notes/bug-2080436-568b03b5b5ba5760.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixes a regression for live migration on shared storage that
+    was removing the backing disk and instance folder during the
+    cleanup of a virtual machine post live migration.
+    `bug 2080436
+    <https://bugs.launchpad.net/nova/+bug/2080436>`__ for details.