block: bdrv_append() fixes

block: bdrv_append() fixes

A few fixups for bdrv_append():

The new bs (bs_new) passed into bdrv_append() should be anonymous.  Rather
than call bdrv_make_anon() to enforce this, use an assert to catch when a caller
is passing in a bs_new that is not anonymous.

Also, the new top layer should have its backing_format reflect the original
top's format.

And last, after the swap of bs contents, the device_name will have been copied
down. This needs to be cleared to reflect the anonymity of the bs that was
pushed down.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>