analyze the `vm.repo` et al

[shadeofblue]

• consider async/non-async
• do we want to distinguish between image and image_hash in the function signature

related: #1044

[johny-b]

Suggestion:

# This can create any payload, and is rarely used directly
payload = Payload(**kwargs)

# Creates VM payloads and uses internally Payload()
# (this is a non-async replacement for `vm.repo()`)
payload = Payload.vm(image_hash, **kwargs)

# Other functions might be implemented in the future
payload = Payload.whatever(any, args)

also related: #404

[cryptobench]

#1044 now distinguishes between image_hash and image_name