block: Ignore duplicate or NULL format_name in bdrv_iterate_format

Some block drivers have multiple BlockDriver instances with identical
format_name fields (e.g. gluster, nbd).

Both qemu-img and qemu will use bdrv_iterate_format() to list the
supported formats when a help option is invoked.  As protocols and
formats may register multiple drivers, redundant listings of formats
occur (e.g., "Supported formats: ... gluster gluster gluster gluster ...
").

Since the list of driver formats will be small, this performs a simple
linear search on format_name, and ignores any duplicates.

The end result change is that the iterator will no longer receive
duplicate string names, nor will it receive NULL pointers.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -3601,10 +3601,25 @@ void bdrv_iterate_format(void (*it)(void *opaque, const char *name),
                          void *opaque)
 {
     BlockDriver *drv;
+    int count = 0;
+    const char **formats = NULL;
 
     QLIST_FOREACH(drv, &bdrv_drivers, list) {
+        if (drv->format_name) {
+            bool found = false;
+            int i = count;
+            while (formats && i && !found) {
+                found = !strcmp(formats[--i], drv->format_name);
+            }
+
+            if (!found) {
+                formats = g_realloc(formats, (count + 1) * sizeof(char *));
+                formats[count++] = drv->format_name;
                 it(opaque, drv->format_name);
             }
+        }
+    }
+    g_free(formats);
 }
 
 /* This function is to find block backend bs */