server: Reject -e with too long of a name

server: Reject -e with too long of a name

The NBD protocol requires strings to be capped at 4k; we violated that
if the client requests NBD_OPT_LIST but the command line provided too
long of a string.

The protocol also requires that strings be valid UTF-8, but for now,
we are accepting any byte sequence.

However, we still need another patch before the test is fully
complete: qemu-nbd --list wants to send a valid NBD_OPT_INFO with
length longer than we currently permit.

Signed-off-by: Eric Blake <eblake@redhat.com>