conf: eliminate repetitive code in virDomainPCIAddressGetNextSlot()

conf: eliminate repetitive code in virDomainPCIAddressGetNextSlot()

virDomainPCIAddressGetNextSlot() starts searching from the last
allocated address and goes to the end of all the buses, then goes back
to the first bus and searches from there up to the starting point (in
case any address has been freed since the last time an address was
allocated. The loops for these two are almost, but not exactly, the
same, so they have remained as separate loops with the same code
inside the loop. To lessen maintenance headaches, the identical code
has been moved out into the function
virDomainPCIAddressFindUnusedFunctionOnBus(), which is called in place
of the loop contents.