When virDBusMessageRead() and virDBusMessageDecode were first added in
commit 834c9c94, they were identical except that virDBusMessageRead()
would unref the message after decoding it.

This difference was eliminated later in commit dc7f3ffc after it
became apparent that unref-ing the message so soon was never the right
thing to do. The two identical functions remained though, with the
tests and virDBus library itself calling the Decode variant, and all
other users calling the Read variant.

This patch eliminates the duplication, switching all users to
virDBusMessageDecode (and moving the nice API documentation comment
from the Read function up to the Decode function).
Signed-off-by: NLaine Stump <laine@laine.org>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>