Add ATTRIBUTE_FALLTHROUGH for switch cases without break

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>