serve nbd: query the disk size using stat()
serve nbd: query the disk size using stat()

This allows a Linux nbd-client to mount the volume and then create
filesystems on it.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

simonjbeaumont/vhd-tool,djs55/vhd-tool,thomassa/vhd-tool,euanh/vhd-tool,euanh/vhd-tool,djs55/vhd-tool