nbd: Less allocation during NBD_OPT_LIST

nbd: Less allocation during NBD_OPT_LIST

Since we know that the maximum name we are willing to accept
is small enough to stack-allocate, rework the iteration over
NBD_OPT_LIST responses to reuse a stack buffer rather than
allocating every time.  Furthermore, we don't even have to
allocate if we know the server's length doesn't match what
we are searching for.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1476469998-28592-12-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>