The NBD protocol has several constants defined in various extensions
that we are about to implement.  Expose them to the code, along with
an easy way to map various constants to strings during diagnostic
messages.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20170707203049.534-4-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>