virsh.setmem: Case extend to cover more functional scenarios

Signed-off-by: Yanbing Du <ydu@redhat.com>
virsh.setmem: Fix the expected memory value

1. After do 'save', 'managedsave' and 'S4' actions, the VM will shutoff
and then VM memory will keep the original state after restore or
start it. It only take effect after restart VM. And, the VM memory
settle action should done before these actions.
2. Fix the check logic for old version libvirt.
Merge pull request #346 from will-Do/setmem

virsh.setmem: Case extend to cover more functional scenarios