ivshmem-server: use a uint16 for client ID

In practice, the number of VM is limited to MAXUINT16 in ivshmem, so use
the same limit on the server (removes a theorical infinite loop)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
