s390x: refactor reset/reipl handling

Calling pause_all_vcpus()/resume_all_vcpus() from a VCPU thread might
not be the best idea. As pause_all_vcpus() temporarily drops the qemu
mutex, two parallel calls to pause_all_vcpus() can be active at a time,
resulting in a deadlock. (either by two VCPUs or by the main thread and a
VCPU)

Let's handle it via the main loop instead, as suggested by Paolo. If we
would have two parallel reset requests by two different VCPUs at the
same time, the last one would win.

We use the existing ipl device to handle it. The nice side effect is
that we can get rid of reipl_requested.

This change implies that all reset handling now goes via the common
path, so "no-reboot" handling is now active for all kinds of reboots.

Let's execute any CPU initialization code on the target CPU using
run_on_cpu.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180424101859.10239-1-david@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

include/hw/s390x/s390-virtio-ccw.h

@@ -53,6 +53,4 @@ bool cpu_model_allowed(void);
  */
 bool css_migration_enabled(void);
 
-void subsystem_reset(void);
-
 #endif