[Libguestfs] [PATCH libnbd] api: Get rid of nbd_connection.

This isn't quite finished because not all of the tests or examples have been updated, but it demonstrates an idea: Should we forget about the concept of having multiple connections managed under a single handle? In this patch there is a single ‘struct nbd_handle *’ which manages a single state machine and connection (and therefore no nbd_connection). To connect to a multi-conn server you must do something like: - Connect once, check if the server supports multi-conn. - If it does, either disconnect and connect mulitple times "for real", or else open further handles as required. An example of use of the new API can be found in examples/threaded-reads-and-writes.c after this patch is applied. Rich.