cmdlib: Merge duplicated code for TLMigrateInstance

LUInstanceFailover and LUInstanceMigrate use TLMigrateInstance and had
the essentially same code for expanding names and declaring locks. In
LUInstanceMigrate.ExpandNames there was a mistake which led to node
resource locks not being declared properly. The two DeclareLocks methods
were exactly the same, down to the byte.

TLMigrateInstance had a lot of keyword parameters. Since it is only used
on two places, all parameters were made positional, making it easier to
determine the value of a parameter.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>