We add helper function colo_supported() to indicate whether
colo is supported or not, with which we use to control whether or not
showing 'x-colo' string to users, they can use qmp command
'query-migrate-capabilities' or hmp command 'info migrate_capabilities'
to learn if colo is supported.

The default value for COLO (COarse-Grain LOck Stepping) is disabled.

Cc: Juan Quintela <quintela@redhat.com>
Cc: Amit Shah <amit.shah@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NAmit Shah <amit@amitshah.net>