pci: missing documentation for dealing with 64 bit config fields

pci spec says what width access to use for 32, 16 and 8
bit fields, but does not explicitly say what to do for
32 bit fields. As we have text that says driver must
treat 64 bit accesses as non-atomic, this seems
to imply driver should always do two 32 bit wide accesses.

Let's make this an explicit requirement, and require
devices to support this.

VIRTIO-139

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio/branches/v1.0@508 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652

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