hypervisors: change MigrateInstance API

Currently the $hypervisor.MigrateInstance takes the instance name. This
patch changes it to take the instance object, such that other instance
properties (especially hvparams) are available to it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>