Add use_localtime parameter for xen-hvm and kvm

Currently xen-hvm and kvm use different real time clock by default. To
reduce confusion, this patch adds an optional use_localtime parameter.

If the real time clock on the instance is set to local time, the
parameter use_localtime should be True. The default is False. Note that
the real time clock changes from local to UTC in xen-hvm with the
default parameter.

Signed-off-by: Jun Futagawa <jfut@integ.jp>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>