docs: Explain how to use passt

docs: Explain how to use passt

Add a chapter to explain how to use passt(1) instead of '-net user'.
passt(1) can be connected to QEMU using UNIX socket or vhost-user.
With vhost-user, migration of the VM is allowed and internal state of
passt(1) is transfered from one side to the other

Bug: https://gitlab.com/qemu-project/qemu/-/issues/2827
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>