COLO: Add 'x-colo-lost-heartbeat' command to trigger failover

COLO: Add 'x-colo-lost-heartbeat' command to trigger failover

We leave users to choose whatever heartbeat solution they want,
if the heartbeat is lost, or other errors they detect, they can use
experimental command 'x_colo_lost_heartbeat' to tell COLO to do failover,
COLO will do operations accordingly.

For example, if the command is sent to the Primary side,
the Primary side will exit COLO mode, does cleanup work,
and then, PVM will take over the service work. If sent to the Secondary side,
the Secondary side will run failover work, then takes over PVM's service work.

Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
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>