librbd/migration/NBDStream: switch to NBD URIs

librbd/migration/NBDStream: switch to NBD URIs

This removes the constraint on the transport being TCP, allowing to
use a Unix domain socket or other options.  It also allows specifying
export names which a) are needed in case of serving different content
on different exports and b) some servers may require regardless.

Additionally, NBD URIs are future proof as all that NBDStream needs to
do is forward the string to libnbd.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>