xenapi: strip base_mirror after live-migrate

There is a bug in XenServer CA-98606 that means you get base_mirror key
left over after a block live-migrate.

This change ensures to strip out that key after error live migrate, to
ensure the key is never left around in a way that means the next live
migrate would fail.

Fixes bug 1234752
Change-Id: Icd6ba17039b50ae20d252d7c052120abe7de0deb
Merge "xenapi: strip base_mirror after live-migrate"