storage physical_block_size is restricted to uint16_t

It is desirable to set -global scsi-hd.physical_block_size=4194304 for RBD-backed storage.

But unfortunatelly, this values is restricted with uint16_t, i.e. 65536 maximum.

For example, scsi-hd.discard_granularity=4194304 is not so restricted (and works as expected)