kvm tools: Document steps to debug kernel using KGDB

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>

diff --git a/tools/kvm/Documentation/kernel-debugging.txt b/tools/kvm/Documentation/kernel-debugging.txt
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+This document explains how to debug a guests' kernel using KGDB.
+
+1. Run the guest:
+        'kvm run -k [vmlinuz] -p "kgdboc=ttyS1 kgdbwait" --tty 1'
+
+And see which PTY got assigned to ttyS1 (you'll see:
+'  Info: Assigned terminal 1 to pty /dev/pts/X').
+
+2. Run GDB on the host:
+        'gdb [vmlinuz]'
+
+3. Connect to the guest (from within GDB):
+        'target remote /dev/pty/X'
+
+4. Start debugging! (enter 'continue' to continue boot).