Modify imageformat according to file used

[gbraad]

https://github.com/code-ready/machine-driver-hyperkit/blob/c592b26d73624e85e1bdb7d962c6f0cea99740e1/pkg/hyperkit/driver.go#L228-L234

When raw imageformat is to be used, the driver needs to handle either a single format or infer this from the file referenced. Related to crc-org/snc#206

[cfergeau]

crc-org/machine#39 adds a VMDriver.ImageFormat field which would make this easier to implement.