So far we were not testing virtio-9p in Travis yet, since we forgot to
install libcap-devel. Do it now to get some more test coverage.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190905113346.2473-1-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>