Convert blkdebug.txt to rST format.  We put it into index-build.rst
because it falls under the "test" part of "QEMU Build and Test
System".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20240816132212.3602106-2-peter.maydell@linaro.org