core: minor cleanup in migration related classes

- Remove redundant setting of migrating-to-vds field in MigrateVmCommand
- Remove '_' prefix in MigrateVDSCommandParameters field names
- Fix indentation in MigrateVDSCommand
- Extract the invocation of MigrateVDSCommand from
  MigrateVmCommand#perform to separate method in order to make the
  perform method more readable

Change-Id: If7e8f60d8e9accf2618ed10fa17f74e56e0d1ae9
Signed-off-by: Arik Hadas <ahadas@redhat.com>