engine: Examine response to check connectivity

The patch examines the result of the response in order
to determine if connectivity to VDSM is established rather on
relying on an exception to be thrown in case of a connectivity
failure.

Change-Id: I8712f5a5d22d2acf6632a4915c6bbed215caeb42
Signed-off-by: Moti Asayag <masayag@redhat.com>