tools/libxl: Add back channel to allow migration target send data back

tools/libxl: Add back channel to allow migration target send data back

In COLO mode, secondary needs to send the following data to primary:
1. In libxl
   Secondary sends the following CHECKPOINT_CONTEXT to primary:
   CHECKPOINT_SVM_SUSPENDED, CHECKPOINT_SVM_READY and CHECKPOINT_SVM_RESUMED
2. In libxc
   Secondary sends the dirty pfn list to primary

But the io_fd only can be written in primary, and only can be read in
secondary. Save recv_fd in domain_suspend_state, and send_fd in
domain_create_state. Extend libxl_domain_create_restore API, add a
send_fd param to it. Add LIBXL_HAVE_CREATE_RESTORE_SEND_FD to indicate
the API change.

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: Wei Liu <wei.liu2@citrix.com>