tests: Distribute virnetserverdata
    
    Fairly recently we've introduced virnetservertest. This test has some
    input data stored under tests/virnetserverdata which unfortunately was
    not distributed among with the test. Therefore 'make distcheck'
    failed. Fix this by adding the directory into EXTRA_DIST.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>