Update KVM guide to v4+ "Build on openSUSE".

[phillxnet]

Thanks to @FroggyFlox for highlighting that our KVM guide has v3 screenshots. Our installer is now very different and it would be less confusing to simply update this reference to relate to our v4+ offering given the deprication of v3 (CentOS based). It is proposed that this would be preferred to having both a v3 and a v4+ variant of this same doc entry:

https://rockstor.com/docs/howtos/kvm_setup.html#kvmsetup

The existing VMM screen shorts are not so dissimilar to newer versions as to warrant an update so can likely be left as-is. Although changing the base OS indicated in these VMM screenshots would be advisable.