BlockSizeConstraints are not yet enforced by the server

[Merovius]

Eventually, the server should enforce block size constraints. We might return errors if the client does not negotiate block sizes, or alternatively do the chunking and aligning ourselves - either way, an nbd.Device should be able to rely on alignment guarantees when it specifies constraints.

Relevant spec values:

• NBD_INFO_BLOCK_SIZE
• NBD_REP_ERR_BLOCK_SIZE_REQD