Change cpuType default to null

[afbjorklund]

Avoid issues with converting null strings in a map value

Change order to alphabetical, to match the "limactl info"

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Helps with:

• https://github.com/lima-vm/lima/pull/1069

• https://github.com/lima-vm/lima/pull/2306

[afbjorklund]

Note: the -pdpe1gb is only true when running on a Darwin host, not on Linux (and looks a bit odd).

But I figured it is best to have details in the code, and avoid duplicating them here... Maybe remove it?

cpuType:
  # aarch64: "cortex-a72" # (or "host" when running on aarch64 host)
  # armv7l: "cortex-a7" # (or "host" when running on armv7l host)
  # riscv64: "rv64" # (or "host" when running on riscv64 host)
  # x86_64: "qemu64" # (or "host" when running on x86_64 host)