block/nbd-client: refactor request send/receive

Add nbd_co_request, to remove code duplications in
nbd_client_co_{pwrite,pread,...} functions. Also this is
needed for further refactoring.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20170804151440.320927-8-vsementsov@virtuozzo.com>
[eblake: make nbd_co_request a wrapper, rather than merging two
existing functions]
Signed-off-by: Eric Blake <eblake@redhat.com>