xenstored: fix possible, but unlikely, stack overflow

...when reading xenbus port from xenfs.

Coverity-ID: 1055741
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/xenstore/xenstored_linux.c

@@ -32,7 +32,7 @@ evtchn_port_t xenbus_evtchn(void)
 	if (fd == -1)
 		return -1;
 
-	rc = read(fd, str, sizeof(str)); 
+	rc = read(fd, str, sizeof(str) - 1);
 	if (rc == -1)
 	{
 		int err = errno;