Make unshelve use new style BDM

Unshelving an instance requires recreating it, and since change
I5b9c3e2d959c602fa22f49db681da918ae0adcea, it is necessary to call
compute manager's _prep_block_device with new style BDMs.

This patch makes the conductor return new-style BDMs when unshelving the
instance.

Closes-bug: 1246412
blueprint: clean-up-legacy-block-device-mapping

Change-Id: I0ba9f53c31c4d5238319cfd09600ad64f298629e
(cherry picked from commit 7fb9fcc970324236267d893185b421e26b1cb1d9)
Merge "Make unshelve use new style BDM" into stable/havana