docs: Add docs/devel/testing.rst

To make our efforts on QEMU testing easier to consume by contributors,
let's add a document. For example, Patchew reports build errors on
patches that should be relatively easy to reproduce with a few steps, and
it is much nicer if there is such a documentation that it can refer to.

This focuses on how to run existing tests and how to write new test
cases, without going into the frameworks themselves.

The VM based testing section is moved from tests/vm/README which now
is a single line pointing to the new doc.

Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20180201022046[email protected]>
Signed-off-by: Fam Zheng <[email protected]>