qemu: fix RTC_CHANGE event for <clock offset='variable' basis='utc'/>
    
    commit e31b5cf393857 attempted to fix libvirt's
    VIR_DOMAIN_EVENT_ID_RTC_CHANGE, which is documentated to always
    provide the new offset of the domain's real time clock from UTC. The
    problem was that, in the case that qemu is provided with an "-rtc
    base=x" where x is an absolute time (rather than "utc" or
    "localtime"), the offset sent by qemu's RTC_CHANGE event is *not* the
    new offset from UTC, but rather is the sum of all changes to the
    domain's RTC since it was started with base=x.
    
    So, despite what was said in commit e31b5cf393857, if we assume that
    the original value stored in "adjustment" was the offset from UTC at
    the time the domain was started, we can always determine the current
    offset from UTC by simply adding the most recent (i.e. current) offset
    from qemu to that original adjustment.
    
    This patch accomplishes that by storing the initial adjustment in the
    domain's status as "adjustment0". Each time a new RTC_CHANGE event is
    received from qemu, we simply add adjustment0 to the value sent by
    qemu, store that as the new adjustment, and forward that value on to
    any event handler.
    
    This patch (*not* e31b5cf393857, which should be reverted prior to
    applying this patch) fixes:
    
    https://bugzilla.redhat.com/show_bug.cgi?id=964177
    
    (for the case where basis='utc'. It does not fix basis='localtime')