Get notified immediately when a balloon target is set, instead of waiting for
up to one second.

The up-to 1 second gap could be long enough to cause swapping inside of the
VM that receives the VM.
Acked-by: NAndy King <acking@vmware.com>
Signed-off-by: NXavier Deguillard <xdeguillard@vmware.com>
Tested-by: NSiva Sankar Reddy B <sankars@vmware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>