qemu: replace calls to virDomainPCIAddressReserveNext*() with static function

qemu: replace calls to virDomainPCIAddressReserveNext*() with static function

An upcoming commit will remove the "flag" argument from all the calls
to reserve the next available address|slot, but I don't want to change
the arguments in the hypervisor-agnostic
virDomainPCIAddressReserveNext*() functions, so this patch places a
simple qemu-specific wrapper around those functions - the new
functions don't take a flags arg, but grab it from the device's
info->pciConnectFlags.