In virDomainVirtioSerialAddrNext, add another controller
if we've exhausted all ports of the existing controllers.

https://bugzilla.redhat.com/show_bug.cgi?id=1076708

Create a sorted array of virtio-serial controllers.
Each of the elements contains the controller index
and a bitmap of available ports.

Buses are not tracked, because they aren't supported by QEMU.

It returns NULL on failure. Checking if the negation of it
is less than zero makes no sense. (Found by coverity after moving
the code)

In another case, the return value wasn't checked at all.

Just code movement and rename.

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.