migration: Document handling of bdrv_is_allocated() errors

migration: Document handling of bdrv_is_allocated() errors

Migration is the only code left in the tree that does not react
to bdrv_is_allocated() failures.  But as there is no useful way
to react to the failure, and we are merely skipping unallocated
sectors on success, just document that our choice of handling
is intended.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>