Windows instances require the timezone to be "localtime"

This was relatively simple to fix, assuming that the image used
to boot the windows instance has the os_type set to "windows".
The use of os_type in this manner is consistent with the
existing use in the Xen hypervisor driver.

DocImpact: if you're booting windows instances, then you need to
set the os_type image property in glance to "windows". Otherwise
your instances will have their clock timezone set to UTC, which
has unexpected side effects in windows.

(cherry picked from commit 280a336a07006b2c998719f2f9033d43289ec7ef)

Conflicts:
	nova/tests/virt/libvirt/test_libvirt.py
	nova/virt/libvirt/driver.py

Change-Id: Id6759a290ed25c9add97ac61bb7d165b3fb908b2
Closes-Bug: 1231254