Hyper-V: Change variable in debug log message

The variable used in the log message is incorrect. The log message
outputs the file size instead of the internal VHD size.

Closes-bug #1275805

(cherry-picked from commit 7020f6eccfd1fb7d6b52e77c363c58815915240a)

Change-Id: I270709098e058cf51bbe6494b903a461b3938d15