QString: Introduce qstring_from_substr()

Note that we can now write qstring_from_str() as a wrapper.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/qstring.h b/qstring.h
index c065331..6aaa7d5 100644
--- a/qstring.h
+++ b/qstring.h
@@ -13,6 +13,7 @@
 
 QString *qstring_new(void);
 QString *qstring_from_str(const char *str);
+QString *qstring_from_substr(const char *str, int start, int end);
 const char *qstring_get_str(const QString *qstring);
 void qstring_append_int(QString *qstring, int64_t value);
 void qstring_append(QString *qstring, const char *str);