rtc: Only call rtc_set_cmos when Register B SET flag is disabled.

This bug occurs when the SET flag of Register B is enabled. When an RTC
data register (i.e. any of the ten time/calender CMOS bytes) is set, the
data is (as expected) correctly stored in the cmos_data array. However,
since the SET flag is enabled, the function rtc_set_time is not invoked.
As a result, the field base_rtc in RTCState remains uninitialized. This
causes a problem on subsequent writes which can end up overwriting data.
To see this, consider writing data to Register A after having written
data to any of the RTC data registers; the following figure illustrates
the call stack for the Register A write operation:

 +- cmos_io_port_write
 +-- check_update_timer
 +---- get_next_alarm
 +------ rtc_update_time

In rtc_update_time, get_guest_rtc calculates the wrong time and
overwrites the previously written RTC data register values.

Signed-off-by: Alex Horn <alex.horn@cs.ox.ac.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
