virconf: fix off-by-1 when appending \n to config file
    
    If the config file does not end with a \n, the parser will append
    one. When re-allocating the array though, it is mistakenly assuming
    that 'len' is the length including the trailing NUL, but it does
    not. So we must add 2 to len, when reallocating, not 1.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>