Ignore non-existent units

In systemd, if a non-existent unit is referenced by other unit, it will
be listed in ListUnits response. This commit ignores these non-existent
entries to prevent performing operations on them.

Tested:
In dropbear.socket, there is a Conflicts=dropbear.service which creates
a non-existent service entry. With this commit, it will not be added to
the service object path in /etc/srvcfg-mgr.json.

Change-Id: I2ce360ea3a0b10f527672d1601ae4d6234c1a9aa
Signed-off-by: Jiaqing Zhao <jiaqing.zhao@intel.com>
diff --git a/inc/utils.hpp b/inc/utils.hpp
index dd2124a..1499588 100644
--- a/inc/utils.hpp
+++ b/inc/utils.hpp
@@ -44,6 +44,7 @@
 static constexpr const char* stateDisabled = "disabled";
 static constexpr const char* subStateRunning = "running";
 static constexpr const char* subStateListening = "listening";
+static constexpr const char* loadStateNotFound = "not-found";
 
 using ListUnitsType =
     std::tuple<std::string, std::string, std::string, std::string, std::string,