Implement the new live migration backend functions

MigrationInfo, AcceptInstance and AbortMigration are implemented as
hypervisor specific functions, and by default they do nothing (as
they're not always necessary).

This patch also converts hv_base.MigrateInstance docstring to epydoc,
adds a missing @type to the GetInstanceInfo docstring, and removes an
unneeded empty line.

Reviewed-by: iustinp