migration: Switch to COLO process after finishing loadvm

Switch from normal migration loadvm process into COLO checkpoint process if
COLO mode is enabled.

We add three new members to struct MigrationIncomingState,
'have_colo_incoming_thread' and 'colo_incoming_thread' record the COLO
related thread for secondary VM, 'migration_incoming_co' records the
original migration incoming coroutine.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Amit Shah <amit@amitshah.net>
