Support colo mode for qemu disk

Support colo mode for qemu disk

Usage: disk = ['...,colo,colo-host=xxx,colo-port=xxx,colo-export=xxx,active-disk=xxx,hidden-disk=xxx...']
For QEMU block replication details:
http://wiki.qemu.org/Features/BlockReplication

Note: we just introduce COLO framework, but don't implement COLO
operations in this patch.

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