Add vm factory support per design in the VM Factory plugin section.
The vm factory controls how a new vm is created:
1. direct: vm is created directly
2. template: vm is created via vm template. A template vm is pre-created
and saved. Later vm is just a clone of the template vm so that they
readonly share a portion of initial memory (including kernel, initramfs
and the kata agent). CPU and memory are hot plugged when necessary.
3. cache: vm is created via vm caches. A set of cached vm are pre-created
and maintained alive. New vms are created by just picking a cached vm.
CPU and memory are hot plugged when necessary.
Fixes: #303
Signed-off-by: Peng Tao <bergwolf@gmail.com>
