Missing virtio drivers after genkernel upgrade on Gentoo
After upgrading genkernel to version 4.0.0_rc1 or higher on Gentoo you may encounter that your KVM virtual machine doesn't boot with a newly built kernel or doesn't have the networking or something else. The reason is that virtio drivers are now disabled by default in genkernel's kernel config.
The solution is to either make use of --virtio command line option or manually enable all needed drivers in the kernel config.
The same also applies to bcache - use --bcache or manually enable CONFIG_BCACHE and to HyperV in genkernel 4.0.0 or higher - use --hyperv or manually enable all needed drivers.