Allow more than 4 virtio-console serial ports.
Allow more than 4 virtio-console serial ports.

As these are PCI devices, the guest can handle and dynamically discover
more of these devices.

Test: create a Cuttlefish device with 18 serial ports, check the device IDs
Bug: b/170149708
Change-Id: Ia2ad73b3a05ee885ce222917c72a53b423fde7c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2450455
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Cody Schuffelen <7618f12f8c1ef40a3c8bf8909fc72185c1ec8ce8@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm