[ovirt] fix waiting for VM to be stopped

The vm.stopped? method on ovirt was not behaving correctly, as it was
returning `true` also when the state was `powering_down`.

I've noticed this when fixing reboot, that also need to wait for vm
to be stopped, and even when adding `stopped?`, it was still failing
with:

[Operation Failed: [Cannot run VM because the VM is in Powering Down
status.]

lib/fog/ovirt/models/compute/server.rb

@@ -44,7 +44,7 @@ module Fog
         end
 
         def stopped?
-          !!(status =~ /down/i)
+          status.downcase == 'down'
         end
 
         def mac
@@ -134,7 +134,10 @@ module Fog
         end
 
         def reboot(options = {})
-          stop unless stopped?
+          unless stopped?
+            stop
+            wait_for { stopped? }
+          end
           start options.merge(:blocking => true)
         end