vl, qapi: offset calculation in RTC_CHANGE event reverted

vl, qapi: offset calculation in RTC_CHANGE event reverted

Return value of qemu_timedate_diff(), used for calculation offset in
QAPI 'RTC_CHANGE' event, restored to keep compatibility. Since it
wasn't documented that difference is relative to host clock
advancement, this change also adds important note to 'RTC_CHANGE'
event description to highlight established implementation specifics.

Signed-off-by: Artem Pisarenko <artem.k.pisarenko@gmail.com>
Message-Id: <1fc12c77e8b7115d3842919a8b586d9cbe4efca6.1539846575.git.artem.k.pisarenko@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>