1. xl has no commands such as start and clone, which is confusing. How can I bring a vm back after I shutdown it?

2. Each time I use 'xl shutdown domid' it comes into "PV control interface not available: external graceful shutdown not possible.

Use "-F" to fallback to ACPI power event." The only way to do it is to use -F option and then the state of the vm turns to block. And it can never be started again. 

3. If I want to use virt-manger with xl. How can I import a vm created with xl into virt-manager? I think there should be a convertion form cfg file to xml file, but I haven't found official solution. What I did is to create a new vm in virt-manager with the disk created by xl. but it always comes into error when I tried to manage vm with virt-manager.

4. how can I change back to use virsh instead of xl as the toolstack to manage xen? It's really aboring to counter all kinds of problems