qmp: Use generic errors in memchar-read, memchar-write

New errors should be generic unless there's a real use case for rich
errors.

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/qemu-char.c b/qemu-char.c
index b593c50..2bdd1bb 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -2764,7 +2764,7 @@
 
     chr = qemu_chr_find(device);
     if (!chr) {
-        error_set(errp, QERR_DEVICE_NOT_FOUND, device);
+        error_setg(errp, "Device '%s' not found", device);
         return;
     }
 
@@ -2799,7 +2799,7 @@
 
     chr = qemu_chr_find(device);
     if (!chr) {
-        error_set(errp, QERR_DEVICE_NOT_FOUND, device);
+        error_setg(errp, "Device '%s' not found", device);
         return NULL;
     }