rtc: Use time_t for passing and returning time offsets

rtc: Use time_t for passing and returning time offsets

The functions qemu_get_timedate() and qemu_timedate_diff() take
and return a time offset as an integer. Coverity points out that
means that when an RTC device implementation holds an offset
as a time_t, as the m48t59 does, the time_t will get truncated.
(CID 1507157, 1517772).

The functions work with time_t internally, so make them use that type
in their APIs.

Note that this won't help any Y2038 issues where either the device
model itself is keeping the offset in a 32-bit integer, or where the
hardware under emulation has Y2038 or other rollover problems.  If we
missed any cases of the former then hopefully Coverity will warn us
about them since after this patch we'd be truncating a time_t in
assignments from qemu_timedate_diff().)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>