[Merge]pluggable_vm_snapshot branch

[Edison Su]

Hi All,
    Per discussion on the thread: http://markmail.org/message/ybw2yy5snkvkuc57, 
we decide to use coarse-graind interface to make taking vm snapshot pluggable. 
On pluggable_vm_snapshot, a new interface is added:

public interface VMSnapshotStrategy {
    VMSnapshot takeVMSnapshot(VMSnapshot vmSnapshot);
    boolean deleteVMSnapshot(VMSnapshot vmSnapshot);
    boolean revertVMSnapshot(VMSnapshot vmSnapshot);
    boolean canHandle(VMSnapshot vmSnapshot);
}

Any vendor can implement this interface to customize vm snapshot procedure.
Unit test is added.
If no objection, I'd like to merge it into master. Comments/feedback are 
welcome, thanks!