transitional driver features: fix typos noted by Rusty

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@311 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652
legacy devices: get rid of MUST assume

as Rusty points out MUST assume is not very good requirement.
clarify it.

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@312 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652
message framing: rusty's comments

generic note on message framing
specific requirements listed for net device only

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@313 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652
block: legacy message framing

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@314 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652
legacy message framing: console device

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@315 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652
legacy framing: scsi host

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@316 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652