Fix 'hostuuid: preload data malformed' warning

If the preloaded hostuuid value is invalid and verbose booting is
enabled, a warning is printed.  This printf had two bugs:

1. It was missing a trailing \n character.
2. The malformed UUID is printed with %s even though it is not known
to be NUL-terminated.

This commit adds the missing \n and uses %.*s with the (already known)
length of the preloaded UUID to ensure that we don't read past the end
of the buffer.

Reported by:	kevans
Fixes:		c3188289 Preload hostuuid for early-boot use
MFC after:	3 days

sys/kern/kern_jail.c

@@ -263,8 +263,8 @@ prison0_init(void)
 				(void)strlcpy(prison0.pr_hostuuid, data,
 				    size + 1);
 			} else if (bootverbose) {
-				printf("hostuuid: preload data malformed: '%s'",
-				    data);
+				printf("hostuuid: preload data malformed: '%.*s'\n",
+				    (int)size, data);
 			}
 		}
 	}