qtest: fix hex2nib for capital characters

Signed-off-by: NSergey Fedorov <serge.fdrv@gmail.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qtest.c

@@ -148,7 +148,7 @@ static int hex2nib(char ch)
     } else if (ch >= 'a' && ch <= 'f') {
         return 10 + (ch - 'a');
     } else if (ch >= 'A' && ch <= 'F') {
-        return 10 + (ch - 'a');
+        return 10 + (ch - 'A');
     } else {
         return -1;
     }