block/nbd: refactor nbd connection parameters

block/nbd: refactor nbd connection parameters

We'll need some connection parameters to be available all the time to
implement nbd reconnect. So, let's refactor them: define additional
parameters in BDRVNBDState, drop them from function parameters, drop
nbd_client_init and separate options parsing instead from nbd_open.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20190618114328[email protected]>
Reviewed-by: Eric Blake <[email protected]>
[eblake: Drop useless 'if' before object_unref]
Signed-off-by: Eric Blake <[email protected]>