virt-libvirt:modify function to get id and uuid for virsh managedsave

Hi Chris,Lucas:
I found that in virsh_managedsave.py, the way to get VM's id or uuid is
not very exacting,
so I changed it to use libvirt_vm's function.
Signed-off-by: NLi Yang <liyang.fnst@cn.fujitsu.com>