logging: fix endless loop on EOF

On EOF condition we always have POLLHUP event and read returns
0 thus we never break loop in virLogHandlerDomainLogFileDrain.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/logging/log_handler.c

@@ -461,6 +461,8 @@ virLogHandlerDomainLogFileDrain(virLogHandlerLogFilePtr file)
             if (errno == EINTR)
                 continue;
             return;
+        } else if (len == 0) {
+            return;
         }
 
         if (virRotatingFileWriterAppend(file->file, buf, len) != len)