libvduse: Add VDUSE (vDPA Device in Userspace) library

libvduse: Add VDUSE (vDPA Device in Userspace) library

VDUSE [1] is a linux framework that makes it possible to implement
software-emulated vDPA devices in userspace. This adds a library
as a subproject to help implementing VDUSE backends in QEMU.

[1] https://www.kernel.org/doc/html/latest/userspace-api/vduse.html

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Message-Id: <20220523084611.91-6-xieyongji@bytedance.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>