hmp: make memchar-read escape ASCII control chars except \n and \t

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index 66ec716..4c100d7 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -869,6 +869,8 @@
 @findex ringbuf_read
 Read and print up to @var{size} bytes from ring buffer character
 device @var{device}.
+Certain non-printable characters are printed \uXXXX, where XXXX is the
+character code in hexadecimal.  Character \ is printed \\.
 Bug: can screw up when the buffer contains invalid UTF-8 sequences,
 NUL characters, after the ring buffer lost data, and when reading
 stops because the size limit is reached.