Add REUSE_EXT in _swap_volume call to blockRebase

_swap_volume is called in cases where new volume file has
already been created.  Add VIR_DOMAIN_BLOCK_REBASE_RESUE_EXT
in the flags passed when calling blockRebase to allow
the existing volume file to be written to.

Closes-Bug: #1218057

Change-Id: I704772ba4bfef474fc788d277c99ba016fb7883f
Merge "Add REUSE_EXT in _swap_volume call to blockRebase"