create struct for machine initialization arguments

create struct for machine initialization arguments

This should help us to:
- More easily add or remove machine initialization arguments without
  having to change every single machine init function;
- More easily make mechanical changes involving the machine init
  functions in the future;
- Let machine initialization forward the init arguments to other
  functions more easily.

This change was half-mechanical process: first the struct was added with
the local ram_size, boot_device, kernel_*, initrd_*, and cpu_model local
variable initialization to all functions. Then the compiler helped me
locate the local variables that are unused, so they could be removed.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>