virlog: Fix build breaker with "comparison between signed and unsigned"

Refactor series 0b231195 worked with virLogDestination type which, depending
on the compiler, might be (and probably will be) an unsigned data type.
However, virEnumFromString may return -1 in case of an error. So, when enum
happens to be unsigned, some compilers will naturally complain about foo:
    'if (foo < 0)'

src/util/virlog.c

@@ -1088,7 +1088,7 @@ virLogParseOutput(const char *src)
     char *abspath = NULL;
     size_t count = 0;
     virLogPriority prio;
-    virLogDestination dest;
+    int dest;
     bool isSUID = virIsSUID();
 
     if (!src)