nbd: make server compliant with fixed newstyle spec

nbd: make server compliant with fixed newstyle spec

If the client does not request the fixed new style protocol,
then we should only accept NBD_OPT_EXPORT_NAME. All other
options are only valid when fixed new style has been activated.

The qemu-nbd client doesn't currently request fixed new style
protocol, but this change won't break qemu-nbd, because it
fortunately only ever uses NBD_OPT_EXPORT_NAME, so was never
triggering the non-compliant server behaviour.

Signed-off-by: Daniel P. Berrange <[email protected]>
Message-Id: <1455129674[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>