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.
CA-98950: fix coalescing with small ancestors.

When the virtual size of the coalesce target's ancestors (grandparent & above,
since the parent's size must match the target's) is smaller (due to a resize),
vhd-util coalesce might incorrectly error out with ERANGE when reading a
sector(s) in a partial block beyond the smaller ancestor's size.
Merge pull request #23 from andreil/ca98950

CA-98950: fix coalescing in the presence of smaller ancestors
Remove unused variable
Merge pull request #22 from andreil/xop289

Use UTC for VHD timestamps