virtio-scsi: use virtqueue_map_sg() when loading requests

The VirtQueueElement struct is serialized during migration but the
in_sg[]/out_sg[] iovec arrays are not usable on the destination host
because the pointers are meaningless.

Use virtqueue_map_sg() to refresh in_sg[]/out_sg[] to valid pointers
based on in_addr[]/out_addr[] hwaddrs.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <1438262173-11546-2-git-send-email-stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
