Xenapi shutdown should return True if vm is shutdown

clean and hard shutdown vm methods return a False if the vm
is currently shutdown. It should return a True, so that its
return value can be used to determine if the vm is shutdown
or not.

Fixes: bug #1195587
Change-Id: I758c43d45b15c3ef231e9eee3ecac620a44db824
Merge "Xenapi shutdown should return True if vm is shutdown"