PCI: fold pci_get_pdev{,_by_domain}()

PCI: fold pci_get_pdev{,_by_domain}()

Rename the latter, subsuming the functionality of the former when passed
NULL as first argument.

Since this requires touching all call sites anyway, take the opportunity
and fold the remaining three parameters into a single pci_sbdf_t one.

No functional change intended. In particular the locking related
assertion needs to continue to be kept silent when a non-NULL domain
pointer is passed - both vpci_read() and vpci_write() call the function
without holding the lock (adding respective locking to vPCI [or finding
an alternative to doing so] is the topic of a separate series).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Rahul Singh <rahul.singh@arm.com>
Tested-by: Rahul Singh <rahul.singh@arm.com>