annotations: Improve asset annotation handling

Make `asset.go` the arbiter of asset annotations by removing all asset
annotations lists from other parts of the codebase.

This makes the code simpler, easier to maintain, and more robust.

Specifically, the previous behaviour was inconsistent as the following
ways:

- `createAssets()` in `sandbox.go` was not handling the following asset
  annotations:

    - firmware:
      - `io.katacontainers.config.hypervisor.firmware`
      - `io.katacontainers.config.hypervisor.firmware_hash`

    - hypervisor:
      - `io.katacontainers.config.hypervisor.path`
      - `io.katacontainers.config.hypervisor.hypervisor_hash`

    - hypervisor control binary:
      - `io.katacontainers.config.hypervisor.ctlpath`
      - `io.katacontainers.config.hypervisor.hypervisorctl_hash`

    - jailer:
      - `io.katacontainers.config.hypervisor.jailer_path`
      - `io.katacontainers.config.hypervisor.jailer_hash`

- `addAssetAnnotations()` in the `oci` package was not handling the
  following asset annotations:

    - hypervisor:
      - `io.katacontainers.config.hypervisor.path`
      - `io.katacontainers.config.hypervisor.hypervisor_hash`

    - hypervisor control binary:
      - `io.katacontainers.config.hypervisor.ctlpath`
      - `io.katacontainers.config.hypervisor.hypervisorctl_hash`

    - jailer:
      - `io.katacontainers.config.hypervisor.jailer_path`
      - `io.katacontainers.config.hypervisor.jailer_hash`

This change fixes the bug where specifying a custom hypervisor path via an
asset annotation was having no effect.

Fixes: #1085.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

src/runtime/virtcontainers/virtcontainers_test.go

@@ -28,6 +28,8 @@ const testKernel = "kernel"
 const testInitrd = "initrd"
 const testImage = "image"
 const testHypervisor = "hypervisor"
+const testJailer = "jailer"
+const testFirmware = "firmware"
 const testVirtiofsd = "virtiofsd"
 const testHypervisorCtl = "hypervisorctl"
 const testBundle = "bundle"