docs: update ivshmem device spec

docs: update ivshmem device spec

Add some notes on the parts needed to use ivshmem devices: more specifically,
explain the purpose of an ivshmem server and the basic concept to use the
ivshmem devices in guests.
Move some parts of the documentation and re-organise it.

Signed-off-by: David Marchand <david.marchand@6wind.com>
Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>