libvhd: use UTC for VHD timestamp

libvhd: use UTC for VHD timestamp

[ported from xapi-project/blktap a79ac2c05f9 ("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.

Signed-off-by: Andrei Lifchits <andrei.lifchits@citrix.com>
Andrei no longer works for Citrix but Germano Percossi (ex-colleague of
Andrei) contacted Andrei and confirmed that
  1) this work was written on Citrix time,
  2) it is OK to have Andrei's SoB.

Remove unused variable "tm".

Signed-off-by: Germano Percossi <germano.percossi@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>