vfio-user: support posted writes

Support an asynchronous send of a vfio-user socket message (no wait for
a reply) when the write is posted. This is only safe when no regions are
mappable by the VM. Add an option to explicitly disable this as well.

Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-17-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
4 files changed