migration/colo: Optimize COLO primary node start code path

Optimize COLO primary start path from:
MIGRATION_STATUS_XXX --> MIGRATION_STATUS_ACTIVE --> MIGRATION_STATUS_COLO --> MIGRATION_STATUS_COMPLETED
To:
MIGRATION_STATUS_XXX --> MIGRATION_STATUS_COLO --> MIGRATION_STATUS_COMPLETED
No need to start primary COLO through "MIGRATION_STATUS_ACTIVE".

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>