CP-7657: Implement PCI database merging

Signed-off-by: John Else <john.else@citrix.com>
CP-7657: Add unit tests for Pci_db.merge
CP-7657: Make pci_db non-optional in find_or_create_supported_types

Signed-off-by: John Else <john.else@citrix.com>
CP-7657: Remove hardcoded paths

Signed-off-by: John Else <john.else@citrix.com>
CP-7657: Support NVIDIA PCI database

If the supplemental NVIDIA PCI database is present, open it and merge it
with the base PCI database.

Signed-off-by: John Else <john.else@citrix.com>