cuda: improve date/time read/write

- Allow date/time to be written
- Use qemu_get_timedate() to initialize the clock

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6313 c046a42c-6fe2-441c-8c8c-71466251a162
