qemu-doc: move included files to docs/system

Since qemu-doc.texi is mostly including files from docs/system,
move the existing include files there for consistency.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200228153619.9906-12-peter.maydell@linaro.org
Message-id: 20200226113034.6741-12-pbonzini@redhat.com
[PMM: update MAINTAINERS line for qemu-option-trace.texi]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>