cmdline switches and config vars to control colo-proxy

cmdline switches and config vars to control colo-proxy

Add cmdline switches to 'xl migrate-receive' command to specify
a domain-specific hotplug script to setup COLO proxy.

Add a new config var 'colo.default.agentscript' to xl.conf, that
allows the user to override the default global script used to
setup COLO proxy.

Signed-off-by: Yang Hongyang <hongyang.yang@easystack.cn>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>