PCI: vmd: Switch to pci_bus_find_emul_domain_nr()

PCI: vmd: Switch to pci_bus_find_emul_domain_nr()

The new common domain number allocator can replace the custom allocator
in VMD.

Beyond some code reuse benefits it does close a potential race whereby
vmd_find_free_domain() collides with new PCI buses coming online with a
conflicting domain number. Such a race has not been observed in practice,
hence not tagging this change as a fix.

As VMD uses pci_create_root_bus() rather than pci_alloc_host_bridge() +
pci_scan_root_bus_bridge() it has no chance to set ->domain_nr in the
bridge so needs to manage freeing the domain number on its own.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Szymon Durawa <szymon.durawa@linux.intel.com>
Cc: Nirmal Patel <nirmal.patel@linux.intel.com>
Link: https://patch.msgid.link/20251024224622.1470555-3-dan.j.williams@intel.com