block: Add bdrv_get_specific_info

Add a function for retrieving an ImageInfoSpecific object from a block
driver.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/block.c b/block.c
index d7ca37e..43acaad 100644
--- a/block.c
+++ b/block.c
@@ -3322,6 +3322,15 @@
     return drv->bdrv_get_info(bs, bdi);
 }
 
+ImageInfoSpecific *bdrv_get_specific_info(BlockDriverState *bs)
+{
+    BlockDriver *drv = bs->drv;
+    if (drv && drv->bdrv_get_specific_info) {
+        return drv->bdrv_get_specific_info(bs);
+    }
+    return NULL;
+}
+
 int bdrv_save_vmstate(BlockDriverState *bs, const uint8_t *buf,
                       int64_t pos, int size)
 {