PCI/AER: Use 32 bit PCI domain numbers

The Intel Volume Management Device (VMD) supports 32-bit domain numbers.
To accommodate this, use u32 instead of u16 to store domain numbers.

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