The recent changes to the testsuite to validate exported symbols
flushed out a case of unconditionally exporting symbols that
were only conditionally compiled under HAVE_AVAHI.

* src/Makefile.am (libvirt_net_rpc_server_la_SOURCES): Compile
virnetservermdns unconditionally.
* configure.ac (HAVE_AVAHI): Drop unused automake conditional.
* src/rpc/virnetservermdns.c: Add fallbacks when Avahi is not
present.