build: Don't use code with dbus_message_unref when built without dbus

build: Don't use code with dbus_message_unref when built without dbus

In order to do that, virNodeSuspendSupportsTargetPMUtils() and
virSystemdPMSupportTarget() are created even when pm-utils and dbus
are compiled out, respectively, but in that case returning -2 meaning
"unavailable" (this return code was already used for unavailability
before).  Error is reported in virNodeSuspendSupportsTarget() only if
both functions returned -2, otherwise the error (or success) is properly
propagated up the stack.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>