Add a new qmp command to do checkpoint, query xen replication status

We can call this qmp command to do checkpoint outside of qemu.
Xen colo will need this function.

Signed-off-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Wen Congyang <wencongyang@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>