libvirt_xml: Support suspend mode in guest XML

This function can help to enable suspend mode in guest XML, which is
needed by domain pm suspend operation.

Signed-off-by: Yanbing Du <ydu@redhat.com>

Use the pm in VMBase.__slots__ to implement the set_pm_suspend().

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>