nbd: Support shorter handshake

nbd: Support shorter handshake

The NBD Protocol allows the server and client to mutually agree
on a shorter handshake (omit the 124 bytes of reserved 0), via
the server advertising NBD_FLAG_NO_ZEROES and the client
acknowledging with NBD_FLAG_C_NO_ZEROES (only possible in
newstyle, whether or not it is fixed newstyle).  It doesn't
shave much off the wire, but we might as well implement it.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Alex Bligh <[email protected]>
Message-Id: <1476469998[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>