add support for compressed image uploads

[sudomateo]

The Preparing Images for Import documentation details that images must be in raw format to be uploaded. However, raw images are an order of magnitude larger than compressed images which leads to long upload times particularly for those with low upload bandwidth. Customers have requested support for uploading compressed images (e.g., .raw.xz).

When uploading a compressed image today the user interface shows the following note.

This image might not be bootable

• Bootable markers not found at any block size.
  Expected either “EFI PART” marker at offsets 512 / 2048 / 4096 or “CD001” at offset 0x8001 (for a bootable CD).

Learn more about preparing images for import

This note does not block the user from uploading the compressed image. If an operator proceeds with the upload the modal will show an error once the upload has finished.