s390/cpu: split CPU reset into architectured functions

s390 provides several CPU resets:
- CPU reset, clears interrupts, stop processing, clears TLB, but does
  not touch registers
- initial CPU reset, like CPU reset, but also clears PSW, prefix, FPC,
  timer and control registers. It does not touch gprs, fprs and acrs (!)
- Power on reset: the full monty

wire up CPUClass reset to the full monty, but provide the lesser resets
as part of S390CPUClass.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
s390: Implement load normal reset

kdump on s390 uses a load normal reset to bring the system in a defined
state by doing a subsystem reset. The issuing CPUs will have an initial
CPU reset, all other CPUs will have a CPU reset as defined in POP (no
register content will change).

Implement this as architectured.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
s390: wire up nmi command to raise a RESTART interrupt on S390

There is the 'nmi' command that is used to trigger a guest dump via kdump feature on x86.
s390 uses RESTART interrupt to trigger kdump.
So, this patch provides a mean to use 'nmi' command on s390 to raise RESTART interrupt.

The CPU to receive the RESTART interrupt is the "default" one.

There is an infrastructure to select the "default" CPU using 'cpu' command.
The 'info cpus' command can be used to see which one is the "default".

In order to wire up the RESTART to 'nmi' command we had to:
1. implement the kvm_s390_cpu_restart function by exporting the existing code
2. implement s390_cpu_restart function as kvm-aware wrapper
3. modify the qmp_inject_nmi function to enable (for s390) the scan for
   "default" CPU and call s390_cpu_restart for it;
3. fix some messages.

Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Alexander Graf <agraf@suse.de>