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: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

qemu-img.c

@@ -304,11 +304,11 @@ static int img_create(int argc, char **argv)
             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 @@ static int img_convert(int argc, char **argv)
             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':