eni_vdpa: add vDPA driver for Alibaba ENI

This patch adds a new vDPA driver for Alibaba ENI(Elastic Network
Interface) which is build upon virtio 0.9.5 specification.
And this driver is only enabled on X86 host currently.

Link: https://lore.kernel.org/r/6a9f32c00609af16bbb2ea32e633b3beb1cbf84b.1635493219.git.wuzongyong@linux.alibaba.com
Signed-off-by: Wu Zongyong <wuzongyong@linux.alibaba.com>
Link: https://lore.kernel.org/r/20211026083214.3375383-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de> # fix Kconfig typo
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>