vdpa: Add eventfd for the vdpa callback

[ Upstream commit 5e68470f ]

Add eventfd for the vdpa callback so that user
can signal it directly instead of triggering the
callback. It will be used for vhost-vdpa case.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Message-Id: <20230323053043.35-9-xieyongji@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Stable-dep-of: 02e9e936 ("vhost_vdpa: assign irq bypass producer token correctly")
Signed-off-by: Sasha Levin <sashal@kernel.org>