Multiple spec per PCI alias limitation

Document and the limitation of the PCI in Placement feature that it
does not support [pci]alias configuration where the name of the alias is
repeated. E.g.

[pci]
alias = { "name": "vf1", "product_id":"10ca", "vendor_id":"8086", "device_type":"type-VF"}
alias = { "name": "vf1", "product_id":"f000", "vendor_id":"8086", "device_type":"type-VF"}

This would mean the alias vf1 can be fulfilled from devices with product
id 10ca OR f000. However this OR relationship cannot be encoded to a
single Placement allocation query as Placement does not support
requesting alternative resource classes for a request[2].

This limitation was encoded in the original PCI in Placement
implementation[1] but we missed to mention it in the doc.
This is now fixed.

[1]https://github.com/openstack/nova/blob/0d484ce37d86e989c8abdf57aec5e334f68206ef/nova/objects/request_spec.py#L504-L528
[2]https://docs.openstack.org/api-ref/placement/#list-allocation-candidates

Related-Bug: #2102038
Change-Id: I9dd78b1498f870a4e4c3f26c23d42d105aec0350
(cherry picked from commit c3f392d)

Author: gibizer

doc/source/admin/pci-passthrough.rst

@@ -431,6 +431,12 @@ be added to the resource provider representing the matching PCI devices.
    (Zed) the nova-compute service will refuse to start with such configuration.
    It is suggested to use the PCI address of the device instead.
 
+.. important::
+   While nova supported configuring :oslo.config:option:`pci.alias` where an
+   alias name is repeated and therefore associated to multiple alias
+   specifications, such configuration is not supported when PCI tracking in
+   Placement is enabled.
+
 The nova-compute service makes sure that existing instances with PCI
 allocations in the nova DB will have a corresponding PCI allocation in
 placement. This allocation healing also acts on any new instances regardless of