TLMigrateInstance: restore live migration behavior

Commit b9187ba2 erroneously incorporated parts of the code of
TLMigrateInstance.CheckPrereq into TLMigrateInstance._RunAllocator. As a
result, all migrations performed without the use of an iallocator would end-up
running non-live.

This patch restores the original behaviour.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>