nbd/client: Split out nbd_receive_one_meta_context()

nbd/client: Split out nbd_receive_one_meta_context()

Extract portions of nbd_negotiate_simple_meta_context() to
a new function nbd_receive_one_meta_context() that copies the
pattern of nbd_receive_list() for performing the argument
validation of one reply.  The error message when the server
replies with more than one context changes slightly, but
that shouldn't happen in the common case.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20190117193658[email protected]>