Handle public volumes and otherwise invalid UUIDs.

Public volumes have short UUIDs (which aren't valid 128-bit UUIDs),
so we can't pass them around.  Even if they were valid UUIDs, we
don't handle clearing cached data on them, and they most likely
don't support fallocate(), so don't match them.

Test: manual
Bug: 74132243
Change-Id: Ib855eb869a86392e96ced94a9926c0b32b87e57e