XOP-289: use UTC for VHD timestamps

Currently, the local timezone is factored into VHD timestamps due to the use of
mktime(). This breaks "vhd-util check" for VHDs created in one timezone and
then moved westward, which results in "primary footer invalid: creation time in
future" errors.
Remove unused variable
Merge pull request #22 from andreil/xop289

Use UTC for VHD timestamps