target-s390x: fix s390_cpu_initial_reset

The s390_cpu_initial_reset function zeroes a big part of the CPU state
structure, including CPU_COMMON, and thus the QEMU TLB structure. As
they should not be initialized with zeroes only, we need to call the
tlb_flush to initialize it correctly.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
