blockdev: Fix regression with the default naming of throttling groups

When I/O limits are set for a block device, the name of the throttling
group is taken from the BlockBackend if the user doesn't specify one.

Commit efaa7c4eeb7490c6f37f3 moved the naming of the BlockBackend in
blockdev_init() to the end of the function, after I/O limits are set.
The consequence is that the throttling group gets an empty name.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-stable@nongnu.org
Message-id: af5cd58bd2c4b9f6c57f260d9cfe586b9fb7d34d.1467986342.git.berto@igalia.com
[mreitz: Use existing "id" variable instead of new "blk_id"]
Signed-off-by: Max Reitz <mreitz@redhat.com>
