Move sharable PCI handling functions to domain_addr.[ch], and
change theirs prefix from 'qemu' to 'vir':

 - virDomainPCIAddressAsString;
 - virDomainPCIAddressBusSetModel;
 - virDomainPCIAddressEnsureAddr;
 - virDomainPCIAddressFlagsCompatible;
 - virDomainPCIAddressGetNextSlot;
 - virDomainPCIAddressReleaseSlot;
 - virDomainPCIAddressReserveAddr;
 - virDomainPCIAddressReserveNextSlot;
 - virDomainPCIAddressReserveSlot;
 - virDomainPCIAddressSetFree;
 - virDomainPCIAddressSetGrow;
 - virDomainPCIAddressSlotInUse;
 - virDomainPCIAddressValidate;

The only change here is function names, the implementation itself
stays untouched.

Extract common allocation code from DomainPCIAddressSetCreate
into virDomainPCIAddressSetAlloc.