Use a Linux-style MAINTAINERS file

I make no claims that this is accurate or exhaustive but I think it's a
reasonable place to start.

As the file mentions, the purpose of this file is to give contributors
information about who they can go to with questions about a particular piece of
code or who they can ask for review.

If you sign up for a piece of code and indicate that it's Maintained or
Supported, please be prepared to be responsive to questions about that
subsystem.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
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v1 -> v2
 - Sort alphabetically
 - Copy in instructions from linux MAINTAINERS
 - Fix entries based on review feedback
