Adding missing variable

Missing variable block_device_mapping is added to prepare_disk_for_vminstance method

Change-Id: Ic759df4d53b6d0f454b2a851215aa40402b3053a
Signed-off-by: Gulsum Atici <gulsum.atici@canonical.com>
(cherry picked from commit 13d0232214b1f8db522ee8861741b938ea37bd45)
3 files changed