pci: introduce a type to store a SBDF

pci: introduce a type to store a SBDF

That provides direct access to all the members that constitute a SBDF.
The only function switched to use it is hvm_pci_decode_addr, because
it makes following patches simpler.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>