PCI: vmd: Correct comment: VMD domains start at 0x10000, not 0x1000

VMD domains are allocated starting at 0x10000, not 0x1000 as the comment
said.  Correct the comment and add a reference to the ACPI spec for _SEG.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>