vfio-user: connect vfio proxy to remote server

vfio-user: connect vfio proxy to remote server

Introduce the vfio-user "proxy": this is the client code responsible for
sending and receiving vfio-user messages across the control socket.

The new files hw/vfio-user/proxy.[ch] contain some basic plumbing for
managing the proxy; initialize the proxy during realization of the
VFIOUserPCIDevice instance.

Originally-by: John Johnson <john.g.johnson@oracle.com>
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-3-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>