Shared storage instance migration

Modify LUMigrateInstance and TLMigrateInstance to allow instance migrations for
instances with DTS_EXT_MIRROR disk templates.

Migrations of shared storage instances require either a target node, or an
iallocator to determine the target node. If none is given, the cluster default
iallocator is used.

Locking behaviour: If the iallocator is used, then initially all nodes are
locked and subsequently only the locks on the source node and the target node
selected by the iallocator are retained.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
[iustin@google.com: small changes in cmdlib.py]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>