Vážně díky všem za rozvinutí diskuze... Já do toho koukal už moc dlouho a přestávál jsem v tom cokoliv vidět.
Zřejmě po celou tu dobu funguje vše

kromě testu, kterým jsem to já blbec celou tu dobu testoval (tedy ping
www.seznam.cz -I eth0 - který vrací destination host unreachable)... To je ta lenost zvednou se ze židle a jít k nějaké klientské stanici... Omlouvám se vám.
Každopádně nastavení je nyní takovéto:
[root@srvr etc]$ service network restart
Deaktivuji rozhraní eth0: [ OK ]
Deaktivuji rozhraní eth1: [ OK ]
Deaktivuji rozhraní loopback: [ OK ]
Aktivuji rozhraní loopback: [ OK ]
Aktivuji rozhraní eth0: [ OK ]
Aktivuji rozhraní eth1: [ OK ]
[root@srvr etc]$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
2: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 2c:27:d7:15:57:ee brd ff:ff:ff:ff:ff:ff
inet 192.168.90.2/24 brd 192.168.90.255 scope global eth1
3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP qlen 1000
link/ether 00:1c:c4:7b:78:aa brd ff:ff:ff:ff:ff:ff
inet 192.168.93.2/24 brd 192.168.93.255 scope global eth0
4: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 100
link/[65534]
inet 10.8.0.1 peer 10.8.0.2/32 scope global tun0
5: virbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN
link/ether 52:54:00:c7:cf:9d brd ff:ff:ff:ff:ff:ff
inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0
6: virbr0-nic: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 500
link/ether 52:54:00:c7:cf:9d brd ff:ff:ff:ff:ff:ff
[root@srvr etc]$ ip r
10.8.0.2 dev tun0 proto kernel scope link src 10.8.0.1
10.8.0.0/24 via 10.8.0.2 dev tun0
192.168.93.0/24 dev eth0 proto kernel scope link src 192.168.93.2
192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1
192.168.90.0/24 dev eth1 proto kernel scope link src 192.168.90.2
169.254.0.0/16 dev eth1 scope link metric 1002
169.254.0.0/16 dev eth0 scope link metric 1003
default via 192.168.90.1 dev eth1
[root@srvr etc]$ iptables --list-rules
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A INPUT -i virbr0 -p udp -m udp --dport 53 -j ACCEPT
-A INPUT -i virbr0 -p tcp -m tcp --dport 53 -j ACCEPT
-A INPUT -i virbr0 -p udp -m udp --dport 67 -j ACCEPT
-A INPUT -i virbr0 -p tcp -m tcp --dport 67 -j ACCEPT
-A FORWARD -d 192.168.122.0/24 -o virbr0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -s 192.168.122.0/24 -i virbr0 -j ACCEPT
-A FORWARD -i virbr0 -o virbr0 -j ACCEPT
-A FORWARD -o virbr0 -j REJECT --reject-with icmp-port-unreachable
-A FORWARD -i virbr0 -j REJECT --reject-with icmp-port-unreachable
[root@srvr etc]$ iptables --list-rules -t nat
-P PREROUTING ACCEPT
-P POSTROUTING ACCEPT
-P OUTPUT ACCEPT
-A POSTROUTING -s 192.168.122.0/24 ! -d 192.168.122.0/24 -p tcp -j MASQUERADE --to-ports 1024-65535
-A POSTROUTING -s 192.168.122.0/24 ! -d 192.168.122.0/24 -p udp -j MASQUERADE --to-ports 1024-65535
-A POSTROUTING -s 192.168.122.0/24 ! -d 192.168.122.0/24 -j MASQUERADE
-A POSTROUTING -o eth1 -j MASQUERADE