s390/vtime: Use get_cpu_timer()

Instead of implementing get_vtimer() use get_cpu_timer()
which does the same.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>