ivshmem: Store file descriptor for vhost-user negotiation

ivshmem: Store file descriptor for vhost-user negotiation

If virtio-net driver allocates memory in ivshmem shared memory,
vhost-net will work correctly, but vhost-user will not work because
a fd of shared memory will not be sent to vhost-user backend.
This patch fixes ivshmem to store file descriptor of shared memory.
It will be used when vhost-user negotiates vhost-user backend.

Signed-off-by: Tetsuya Mukawa <mukawa@igel.co.jp>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>