block: Use GString instead of QString to build filenames

QString supports modifying its string, but it's quite limited: you can
only append.  Just one caller remains:
bdrv_parse_filename_strip_prefix() uses it just for building an
initial string.

Change it to do build the initial string with GString.  This is
another step towards making QString immutable.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-20-armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

block.c

@@ -217,7 +217,7 @@ void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
         /* Stripping the explicit protocol prefix may result in a protocol
          * prefix being (wrongly) detected (if the filename contains a colon) */
         if (path_has_protocol(filename)) {
-            QString *fat_filename;
+            GString *fat_filename;
 
             /* This means there is some colon before the first slash; therefore,
              * this cannot be an absolute path */
@@ -225,12 +225,13 @@ void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
 
             /* And we can thus fix the protocol detection issue by prefixing it
              * by "./" */
-            fat_filename = qstring_from_str("./");
-            qstring_append(fat_filename, filename);
+            fat_filename = g_string_new("./");
+            g_string_append(fat_filename, filename);
 
-            assert(!path_has_protocol(qstring_get_str(fat_filename)));
+            assert(!path_has_protocol(fat_filename->str));
 
-            qdict_put(options, "filename", fat_filename);
+            qdict_put(options, "filename",
+                      qstring_from_gstring(fat_filename));
         } else {
             /* If no protocol prefix was detected, we can use the shortened
              * filename as-is */