RTC: Get and set time without going through s->current_tm

RTC: Get and set time without going through s->current_tm

This patch makes rtc_set_time and rtc_set_cmos work without reading
s->current_tm.  In the case of rtc_set_time I introduce a new
function that retrieves the time and stores into a given struct tm
(not hard-coded to s->current_tm).  In the case of rtc_set_cmos, the
current time is similarly taken from a struct tm rather than
s->current_tm.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>