nbd/client: Send NBD_CMD_DISC if open fails after connect

nbd/client: Send NBD_CMD_DISC if open fails after connect

If nbd_client_init() fails after we are already connected,
then the server will spam logs with:

Disconnect client, due to: Unexpected end-of-file before all bytes were read

unless we gracefully disconnect before closing the connection.

Ways to trigger this:

$ opts=driver=nbd,export=foo,server.type=inet,server.host=localhost,server.port=10809
$  qemu-img map --output=json --image-opts $opts,read-only=off
$  qemu-img map --output=json --image-opts $opts,x-dirty-bitmap=nosuch:

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181130023232.3079982-4-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
(cherry picked from commit c688e6ca7b41a105241054853d250df64addbf8f)
*drop functional dep. on 6c2e581d4d7
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>