conf: new function virDomainPCIAddressReserveNextAddr()

conf: new function virDomainPCIAddressReserveNextAddr()

There is an existing virDomainPCIAddressReserveNextSlot() which will
reserve all functions of the next available PCI slot. One place in the
qemu PCI address assignment code requires reserving a *single*
function of the next available PCI slot. This patch modifies and
renames virDomainPCIAddressReserveNextSlot() so that it can fulfill
both the original purpose and the need to reserve a single function.

(This is being done so that the abovementioned code in qemu can have
its "kind of open coded" solution replaced with a call to this new
function).