migration/colo: Fix qmp_xen_colo_do_checkpoint() error handling

migration/colo: Fix qmp_xen_colo_do_checkpoint() error handling

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

qmp_xen_colo_do_checkpoint() passes @errp first to
replication_do_checkpoint_all(), and then to
colo_notify_filters_event().  If both fail, this will trip the
assertion in error_setv().

Similar code in secondary_vm_do_failover() calls
colo_notify_filters_event() only after replication_do_checkpoint_all()
succeeded.  Do the same here.

Fixes: 0e8818f023616677416840d6ddc880db8de3c967
Cc: Zhang Chen <chen.zhang@intel.com>
Cc: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Message-Id: <20200422130719.28225-12-armbru@redhat.com>