Dispose command

[wekesa360]

This PR addresses the VirtualMachine Command Set adding the Dispose command to the python DSL

[michalgr]

Set can be empty, so perhaps we don't need None here ?

1. It's good not to have two ways to represent the same fact: None and set() are two ways to represent that there are no errors associated with a command. It would be unfortunate if some commands used the first and some the second way.
2. it might lead to simpler code processing Commands if error cannot be None. For example, if we wanted to check whether an error code is valid then:

rc in command.error

is simpler than:

True if command.error is None else rc in command.error

At the same time, it's easy to check for there not being any error types:

if command.error:
    print("there are no errors allowed for this command")

[wekesa360]

I have used an empty set: set () for the command with no error.