virutil: Move string related functions to virstring.c

The source code base needs to be adapted as well. Some files
include virutil.h just for the string related functions (here,
the include is substituted to match the new file), some include
virutil.h without any need (here, the include is removed), and
some require both.

tests/libvirtdconftest.c

@@ -29,6 +29,7 @@
 #include "virerror.h"
 #include "virlog.h"
 #include "virconf.h"
+#include "virstring.h"
 
 #define VIR_FROM_THIS VIR_FROM_NONE