vdpa: add vDPA and vhost-vdpa simple description
vdpa: add vDPA and vhost-vdpa simple description

Explained a bit vDPA and vhost-vdpa in the documentation of
the VhostVdpa trait.

Signed-off-by: Stefano Garzarella <6f3dd2004310421c0dda1107f4857e62a774d5df@redhat.com>

rust-vmm/vhost