There is no restriction on maximum value of PCI domain. In fact,
Linux kernel uses plain atomic inc when assigning PCI domains:

drivers/pci/pci.c:static int pci_get_new_domain_nr(void)
drivers/pci/pci.c-{
drivers/pci/pci.c-      return atomic_inc_return(&__domain_nr);
drivers/pci/pci.c-}

Of course, this function is called only if kernel was compiled
without PCI domain support or ACPI did not provide PCI domain.

However, QEMU still has the same restriction as us: in
set_pci_host_devaddr() QEMU checks if domain isn't greater than
0xffff. But one can argue that that's a QEMU limitation. We still
want to be able to cope with other hypervisors that don't have
this limitation (possibly).
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>