9p: move request management to client code

9p: move request management to client code

The virtio transport uses a simplified request management system
that I want to use for all transports.  This patch adapts and moves the
exisiting code for managing requests to the client common code.
Later patches will apply these mechanisms to the other transports.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>