util/qemu-error: prepend guest name to error message to identify affected VM owner

This is followup patch to the one submitted back in Oct, 19

https://lists.gnu.org/archive/html/qemu-devel/2019-10/msg02102.html

My mistake here, I took my eyes of the mailing list after I got the
initial thumbs up. This patch follows up on Markus comments in the
above link.

Purpose of this patch:

We want to print guest name for errors, warnings and info messages. This
was the first of two patches the second being MCE errors targeting a VM
with guest name prepended. But in a large fleet we see many other
errors that disable a VM or crash it. In a large fleet and centralized
logging having the guest name enables identify of owner and customer.

Signed-off-by: Mario Smarduch <msmarduch@digitalocean.com>
Message-Id: <20200626201900.8876-1-msmarduch@digitalocean.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/qemu-options.hx b/qemu-options.hx
index c6edb40..d2c1e95 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4303,16 +4303,22 @@
 DEF("no-kvm", 0, QEMU_OPTION_no_kvm, "", QEMU_ARCH_I386)
 
 DEF("msg", HAS_ARG, QEMU_OPTION_msg,
-    "-msg timestamp[=on|off]\n"
+    "-msg [timestamp[=on|off]][,guest-name=[on|off]]\n"
     "                control error message format\n"
-    "                timestamp=on enables timestamps (default: off)\n",
+    "                timestamp=on enables timestamps (default: off)\n"
+    "                guest-name=on enables guest name prefix but only if\n"
+    "                              -name guest option is set (default: off)\n",
     QEMU_ARCH_ALL)
 SRST
-``-msg timestamp[=on|off]``
+``-msg [timestamp[=on|off]][,guest-name[=on|off]]``
     Control error message format.
 
     ``timestamp=on|off``
         Prefix messages with a timestamp. Default is off.
+
+    ``guest-name=on|off``
+        Prefix messages with guest name but only if -name guest option is set
+        otherwise the option is ignored. Default is off.
 ERST
 
 DEF("dump-vmstate", HAS_ARG, QEMU_OPTION_dump_vmstate,