KVM: Documentation: document how KVM is tested

Proper testing greatly simplifies both patch development and review,
but it can be unclear what kind of userspace or guest support
should accompany new features. Clarify maintainer expectations
in terms of testing expectations; additionally, list the cases in
which open-source userspace support is pretty much a necessity and
its absence can only be mitigated by selftests.

While these ideas have long been followed implicitly by KVM contributors
and maintainers, formalize them in writing to provide consistent (though
not universal) guidelines.

Suggested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>