xenapi: stop server destroy on live_migrate errors

Currently where there are errors after live-migration has completed, the
server is destroyed, which for XenAPI driver, means the server no longer
exists, because XenServer has already deleted the source VM.

Closes-Bug: #1262206
Change-Id: I7f827f5d972da26f5d1edae7a22fb45a1f5dd6ca
Merge "xenapi: stop server destroy on live_migrate errors"