block/vvfat: qcow driver may not be found

Although virtually impossible right now, bdrv_find_format("qcow") may
fail. The vvfat block driver should heed that case.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 1bcb15c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

block/vvfat.c

@@ -2926,6 +2926,12 @@ static int enable_write_target(BDRVVVFATState *s, Error **errp)
     }
 
     bdrv_qcow = bdrv_find_format("qcow");
+    if (!bdrv_qcow) {
+        error_setg(errp, "Failed to locate qcow driver");
+        ret = -ENOENT;
+        goto err;
+    }
+
     opts = qemu_opts_create(bdrv_qcow->create_opts, NULL, 0, &error_abort);
     qemu_opt_set_number(opts, BLOCK_OPT_SIZE, s->sector_count * 512);
     qemu_opt_set(opts, BLOCK_OPT_BACKING_FILE, "fat:");