qemu-img: Don't prepend qemu-img to error messages twice.

error_report() prepends the location already.  Example:

$ qemu-img convert -6
qemu-img: qemu-img: option -6 is deprecated, please use '-o compat6' instead!

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
diff --git a/qemu-img.c b/qemu-img.c
index 1a37828..32628b3 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -304,11 +304,11 @@
             fmt = optarg;
             break;
         case 'e':
-            error_report("qemu-img: option -e is deprecated, please use \'-o "
+            error_report("option -e is deprecated, please use \'-o "
                   "encryption\' instead!");
             return 1;
         case '6':
-            error_report("qemu-img: option -6 is deprecated, please use \'-o "
+            error_report("option -6 is deprecated, please use \'-o "
                   "compat6\' instead!");
             return 1;
         case 'o':
@@ -633,11 +633,11 @@
             compress = 1;
             break;
         case 'e':
-            error_report("qemu-img: option -e is deprecated, please use \'-o "
+            error_report("option -e is deprecated, please use \'-o "
                   "encryption\' instead!");
             return 1;
         case '6':
-            error_report("qemu-img: option -6 is deprecated, please use \'-o "
+            error_report("option -6 is deprecated, please use \'-o "
                   "compat6\' instead!");
             return 1;
         case 'o':