re-visit disable-modern flag that is used across kata

[ganeshmaharaj]

There are a lot of instances where disable-modern is set to be true if we are on a nested environment. https://github.com/kata-containers/runtime/search?q=DisableModern&unscoped_q=DisableModern. The reason for this seems to be some instabilities when using virtio 1.0 with an unknown version of KVM along with qemu-lite 2.11. Please see #1870 for more information. It is time to re-visit all this and see if we still need to keep them disabled. if disable-modern should be an option that users can set, then that can move into a user visible configuration.

[ganeshmaharaj]

Part of it (one that effects hotplugging devices has been removed using #1868. This issue is open to address all the other instances across the code.

[ganeshmaharaj]

the nested checks and disable-modern enabling which were done to address a kvm issue on azure, seems to have been fixed with https://patchwork.kernel.org/patch/10184241/. Make sure this information is included in the pull request fixing this PR. Also make sure we document which base kernel beyond which this issue will not persist.

[ganeshmaharaj]

Issue fixed with torvalds/linux@d391f12 and has landed in kernel 4.16. Any kernel before that will need to disable-modern .