net/virtio: move vhost-user specifics to its backend

net/virtio: move vhost-user specifics to its backend

This patch moves all the Vhost-user backend specific
logic like Vhost FD, listen FD and interrupt handling
to the vhost-user backend implementation.

In order to achieve that, new ops are created to update
the link status, disconnect and reconnect the server,
and fetch the link state interrupt FD.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>