s390x/kvm: Rework SIGP INITIAL CPU RESET handler

The s390_cpu_initial_reset() function had two deficiencies: First, it
used an ioctl for the destination CPU, and this ioctl could block
nearly forever, as long as the destination CPU was running in the SIE
loop. Second, it also cleared the general purpose registers - something
it should not do according to the Principles of Operations.
Since we've already got another function for the initial CPU reset in
cpu.c, we can also use that function instead. And by using run_on_cpu()
for executing this code, we make sure that the destination CPU is
correctly kicked out of kernel mode now.

Suggested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
