Don't send reply messages for non-method calls in QDBusMessage

QDBusMessage is intended to avoid sending reply messages unless
the message is a method call without the NO_REPLY_EXPECTED flag set.
However, since messages which are not method calls will never have
this flag set, the code will currently cause all non-method call
messages to expect a reply.  This patch changes the code to examine
the message type, and to only check for the flag in cases where the
message is a method call.

Change-Id: Ic5bb00df69d3cfb38f60bf6bfd8463fb28cf2c99
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>