Tradeoff: Automatic or explicit

Filler - need to demonstrate: start with 1k image

Automatic (like file system):
NBD_CMD_WRITE(offset=0,len=2k) -> success, size now 2k

Explicit (like block device):
NBD_CMD_WRITE(offset=0,len=2k) -> fails
NBD_CMD_RESIZE(2k) -> success, size now 2k
NBD_CMD_WRITE(offset=0,len=2k) -> success