This change adds support for a separate socket to carry commands in the
server-to-client direction. It has proven problematic to send commands
in both directions over a single socket, since matching replies to
commands can become non-trivial when both sides send commands at the same
time and adds significant complexity. See issue nutanix#279 for details.

To set up the reverse communication channel, the client indicates
support for it via a new capability flag in the version message. The
server will then create a fresh pair of sockets and pass one end to the
client in its version reply. When the server wishes to send commands to
the client at a later point, it now uses its end of the new socket pair
rather than the main socket. Corresponding replies are also passed back
over the new socket pair.

Signed-off-by: Mattias Nissler <mnissler@rivosinc.com>

Many tests have a `sock` global holding the client socket. In order to
allow additional client context to be managed, introduce a Client class
and replace the `sock` global with `client` global object.

Signed-off-by: Mattias Nissler <mnissler@rivosinc.com>

The new tests verify behavior of vfu_sgl_{read,write} for success and
error cases.

Signed-off-by: Mattias Nissler <mnissler@rivosinc.com>

Include rationale and operation of separate server-to-client command
channel in the protocol description and indicate which commands should
be processed on the separate socket.

Signed-off-by: Mattias Nissler <mnissler@rivosinc.com>

…ities.

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