Add ATTRIBUTE_FALLTHROUGH for switch cases without break
    
    In GCC 7 there is a new warning triggered when a switch
    case has a conditional statement (eg if ... else...) and
    some of the code paths fallthrough to the next switch
    statement. e.g.
    
    conf/domain_conf.c: In function 'virDomainChrEquals':
    conf/domain_conf.c:14926:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
             if (src->targetTypeAttr != tgt->targetTypeAttr)
                ^
    conf/domain_conf.c:14928:5: note: here
         case VIR_DOMAIN_CHR_DEVICE_TYPE_CONSOLE:
         ^~~~
    conf/domain_conf.c: In function 'virDomainChrDefFormat':
    conf/domain_conf.c:22143:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
             if (def->targetTypeAttr) {
                ^
    conf/domain_conf.c:22151:5: note: here
         default:
         ^~~~~~~
    
    GCC introduced a __attribute__((fallthrough)) to let you
    indicate that this is intentionale behaviour rather than
    a bug.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>