Fix DTS_EXT_MIRROR migration

Fix DTS_EXT_MIRROR migration

Commit faaabe3c fixed failover behaviour for DTS_INT_MIRROR instances, however
it broke migration for DTS_EXT_MIRROR instances, by moving iallocator and node
checks from LUInstanceMigrate to TLMigrateInstance. This has the side-effect
that the LU called the TL with None for both, node and iallocator when the
default iallocator was being used.

This patch maintains the iallocator checks in TLMigrateInstance and fixes the
LU-TL integration.

Signed-off-by: Iustin Pop <iustin@google.com>
[iustin@google.com: rebased patch on current HEAD]
Reviewed-by: Iustin Pop <iustin@google.com>