block_resize: needs sleep a while when check the disk size after block_resize



Signed-off-by: Cong Li <coli@redhat.com>
Hi @ypu 

1. We expect there is output of the error block size if the block resize failed.
    Then it needs a global variant current_size.
2. The reason why we can't return the current_size from compare_block_size() is that
     wait_for() needs a bool value (True or False).

According to the above info, I want to set a global varaint current_size, but it still needs return. :(