block: Improve driver whitelist checks

The main intent of this patch is to consolidate the whitelist checks to
a single point in the code instead of spreading it everywhere. This adds
a nicer error message for read-only whitelisting, too, in places where
it was still missing.

The patch also contains a bonus bug fix: By finding the format first in
bdrv_open() and then independently checking against the whitelist only
later, we avoid the case that use of a non-whitelisted format results in
probing rather than an error message. Previously, this could happen when
using the driver=... option.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>

blockdev.c

@@ -468,13 +468,9 @@ static DriveInfo *blockdev_init(QemuOpts *all_opts,
             return NULL;
         }
 
-        drv = bdrv_find_whitelisted_format(buf, ro);
+        drv = bdrv_find_format(buf);
         if (!drv) {
-            if (!ro && bdrv_find_whitelisted_format(buf, !ro)) {
-                error_report("'%s' can be only used as read-only device.", buf);
-            } else {
-                error_report("'%s' invalid format", buf);
-            }
+            error_report("'%s' invalid format", buf);
             return NULL;
         }
     }