ivshmem: limit maximum number of peers to G_MAXUINT16

Limit the maximum number of peers to MAXUINT16. This is more realistic
and better matches the limit of the doorbell register.

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