Make sure you are using Java 6 when trying to access your OVM Virtual Machines. Java 7 seems to have broken something. Isn’t it nice that Oracle makes java and breaks its own products?
Use this download link for Java 6
cd /usr/lib64/mozilla/plugins
ln -s /usr/java/jre1.6.0_37/lib/amd64/libnpjp2.so .
restart firefox
got “about:plugins” and make sure it is enabled.
now choose console from OVM open file as /usr/java/latest/java/bin/javaws
alwasy