This reverts commit e31b5cf3.

This commit attempted to work around a bug in the offset value
reported by qemu's RTC_CHANGE event in the case that a variable base
date was given on the qemu commandline. The patch mixed up the math
involved in arriving at the corrected offset to report, and in the
process added an unnecessary private attribute to the clock
element. Since that element is private/internal and not used by anyone
else, it makes sense to simplify things by removing it.