Changes needed in podman-machine so that it can be used to start a crc VM

[cfergeau]

• configurable way to fetch the disk image so that I can tell podman-machine to use a crc bundle instead of fetching an image from quay.io (there's already a todo in the code for this)
• make env.MachineDirs configurable to have more control over where our stuff is going?
• make sure it's not an issue if we don't use ignition. My understanding of the code is that the VM will be provided ignition configuration data, but it's not a problem if it does not consume it
• make configuration/forwarding of the podman connection socket optional? (though crc also does it)
• logging/printing to stdout should be optional

[cfergeau]

See #31 for another list of related problems.