um: virt-pci: fix uapi documentation

The identifier names in the documentation here didn't match
the real ones, and the reserved was missing. Fix that.

Reported-by: Bjorn Helgaas <helgaas@kernel.org>
Fixes: 68f5d3f3

 ("um: add PCI over virtio emulation driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>