This kills off the funny state described in the previous commit.

Simplify ivshmem_io_read() accordingly, and update documentation.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1458066895-20632-27-git-send-email-armbru@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>