We are going to implement non-blocking version of
nbd_establish_connection, which for a while will be used only for
nbd_reconnect_attempt, not for nbd_open, so we need to call it
separately.

Refactor nbd_reconnect_attempt in a way which makes next commit
simpler.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200727184751.15704-2-vsementsov@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>