* qemuDomainPCIAddressSetNextAddr

The name of this function was confusing because 1) other functions in
the file that end in "Addr" are only operating on a single function of
one PCI slot, not the entire slot, while functions that do something
with the entire slot end in "Slot", and 2) it didn't contain a verb
describing what it is doing (the "Set" refers to the set that contains
all PCI buses in the system, used to keep track of which slots in
which buses are already reserved for use).

It is now renamed to qemuDomainPCIAddressReserveNextSlot, which more
clearly describes what it is doing. Arguably, it could have been
changed to qemuDomainPCIAddressSetReserveNextSlot, but 1) the word
"set" is confusing in this context because it could be intended as a
verb or as a noun, and 2) most other functions that operate on a
single slot or address within this set are also named
qemuDomainPCIAddress... rather than qemuDomainPCIAddressSet... Only
the Create, Free, and Grow functions for an address set (which modify the
entire set, not just one element) use "Set" in their name.

* qemuPCIAddressAsString, qemuPCIAddressValidate

All the other functions in this set are named
qemuDomainPCIAddressxxxxx, so I renamed these to be consistent.