libvirt_xml: Support suspend mode in guest XML

For domain pm suspend operation,  it requires a guest agent configured and running in the domain's guest OS, besides, it also needs  enable suspend mode in guest XML:
```
   <pm>
     <suspend-to-mem enabled='yes'/>
     <suspend-to-disk enabled='yes'/>
   </pm>
```
Signed-off-by: Yanbing Du <ydu@redhat.com>