Replace pci_request flavor storage with proper object usage

This makes the PCI code use the InstancePCIRequests object to store
and retrieve the pci_request information, instead of stashing it in
system_metadata.

Related to blueprint pci-passthrough-sriov

Co-Authored-By: Baodong (Robert) Li <baoli@cisco.com>
Change-Id: I012ee5c118265e044ff41fb58b732728946ee85a
Merge "Replace pci_request flavor storage with proper object usage"