[RFC] Create a candidate list for deprecated feature removal

Today Kata containers has grown to have a lot of features and capabilities. Some of these capabilities are options which a user can choose from. These have evolved over time and newer capabilities have replaced older ones for the same feature. This results in code complexity without additional value. The goal of this issue to identify candidates for removal and leave just the ones that work best for a given hypervisor.

This also increases our validation burden.

This will also allows to default to an implementation for a given hypervisor.

For QEMU/NEMU:

• Networking: Bridge mode, macvtap mode and tcmirror
  - Eliminate Bridge mode as tcmirror replaces it
• Storage: virtio-blk, virtio-scsi and virtio-mmio
  - Eliminate virtio-blk (is there any need for it). It is simpler however.
• VM to Host: vsock, virtio-serial
  - Too early to eliminate virtio-serial but if our user base has newer kernels we can consider eliminating serial