On error, virStoragePoolGetAutostart would return -1 leaving autostart
untouched.

Removed the misleading debug message as well.

Error: CHECKED_RETURN (CWE-252):
libvirt-0.10.2/tools/virsh-pool.c:1386: unchecked_value: No check of the
    return value of "virStoragePoolGetAutostart(pool, &autostart)".