Doc: add document for Dragonball API
add detailed explanation for Dragonball API
We provide plenty API for Kata runtime to interact with `Dragonball` virtual machine manager.
This document provides the introduction for each of them.
-TODO: Details will be added in the Part III PR for `Dragonball`
+## `ConfigureBootSource`
+Configure the boot source of the VM using `BootSourceConfig`. This action can only be called before the VM has booted.
+
+### Boot Source Config
+1. `kernel_path`: Path of the kernel image. `Dragonball` only supports compressed kernel image for now.
+2. `initrd_path`: Path of the initrd (could be None)
+3. `boot_args`: Boot arguments passed to the kernel (could be None)
+
+## `SetVmConfiguration`
+Set virtual machine configuration using `VmConfigInfo` to initialize VM.
+
+### VM Config Info
+1. `vcpu_count`: Number of vCPU to start. Currently we only support up to 255 vCPUs.
+2. `max_vcpu_count`: Max number of vCPU can be added through CPU hotplug.
+3. `cpu_pm`: CPU power management.
+4. `cpu_topology`: CPU topology information (including `threads_per_core`, `cores_per_die`, `dies_per_socket` and `sockets`).
+5. `vpmu_feature`: `vPMU` feature level.
+6. `mem_type`: Memory type that can be either `hugetlbfs` or `shmem`, default is `shmem`.
+7. `mem_file_path` : Memory file path.
+8. `mem_size_mib`: The memory size in MiB. The maximum memory size is 1TB.
+9. `serial_path`: Optional sock path.
