Refactor dsl_source_is_live to correctly check for invalid source

[rjhowell44]

The service should return DslReturnType and not the value of the query... change

/**
 * @brief returns whether the source stream is live or not
 * @param[in] name the name of Source component to query
 * @return True if the source's stream is live
 */
boolean dsl_source_is_live(const wchar_t* name);

to

/**
 * @brief returns whether the source stream is live or not
 * @param[in] name the name of Source component to query
 * @param[out] is_live true if the source is a "live" source, false otherwise
 * @return DSL_RESULT_SUCCESS on successful query, one of DSL_RESULT_SOURCE_RESULT otherwise.
 */
DslReturnType dsl_source_is_live(const wchar_t* name, boolean* is_live);