Yesterday I have applied all patches form openSUSE on my KVM servers and desktop machines.

After restart all virtual machines in KVM lost network connectivity, while the server in self still works on LAN without connectivity troubles.
VirtualBix which also uses KVM virtualization, also had lost networking from the virtual machines. But host computer works fine on LAN.
So based on this I am pretty sure KVM server or some from the accompanying it packages are the reason for this problem.

I have tried to ping some virtual machines from outside and some of them respond to ping but some not.
Some machines (windows based and Linux based) could ping them selves but could not ping outside. 
I have one virtual machine where I could not ping in Internet by name but can pring by IP the same address.

Virtual machines in the same network in KVM can not ping each other or access each other by ssh. The response is "no route to that address"

Before the update of the systems every thing worked just fine.
So now all services provided by the virtual machines are not accessible by the users, but their machines are working well in the KVM excluding they can not communicate via LAN.

For me this is critical and I need to fix above troubles promptly.
If you can suggest how to overcome this it will be warmly welcome.

Mean time I will try KVM from Red Hat / CentOS and if it work fine, will be enforced to migrate all virtual machines from openSUSE KVM to Red Hat based KVM.