qemu: remove qemuDomainPCIAddressReserveNextAddr()
    
    This function is only called in two places, and the function itself is
    just adding a single argument and calling
    virDomainPCIAddressReserveNextAddr(), so we can remove it and instead
    call virDomainPCIAddressReserveNextAddr() directly. (The main
    motivation for doing this is to free up the name so that
    qemuDomainPCIAddressReserveNextSlot() can be renamed in the next
    patch, as its current name is now inaccurate and misleading).