cannot use multiple volumes on uc20

[Ian Johnson]

UC20 does not currently support multiple volumes in the gadget.yaml. There are a few blockers:

1. the gadget validation currently explicitly blocks multiple volumes and fails validation
2. Even if modified trivially to remove that, the current gadget validation code really is coded to the assumption that all of the ubuntu-* partitions are on the same volume, when there are valid and useful use-cases to have the ubuntu-seed and ubuntu-save partitions on a different volume from the ubuntu-boot and ubuntu-data partitions for example.
3. We have added cross-checks to UC20 to fix https://bugs.launchpad.net/snapd/+bug/1863886, however these cross-checks require that all of the ubuntu-* partitions be on the same volume/disk. We need to think how to do similar cross-checks from i.e. the initramfs where we don't currently have details about the gadget to securely determine which disk is the right one to mount/use.

[Ian Johnson (anonymouse67)]

We will be working first on unblocking multiple volumes so that things other than the ubuntu-* partitions can be on other volumes, but still keep the requirement that all ubuntu-* partitions be on the same volume, as this is just a plain bug/unimplemented cases in the code, but the other features will be more complex and will take more time to implement.

[Ian Johnson (anonymouse67)]

This work was done to enable using multiple volumes so long as all the ubuntu-* partitions are on a single volume.

We do not yet allow having the ubuntu-* partitions spread across multiple volumes.